#csi asks

Now, it happens sometimes that the bond of friendship turns out to be firmer and more sincere than that in a romantically united couple (especially when one element or both of the couple have already realized they are in love). In my opinion, it was this kind of relationship that set everything in motion.

Superficially, Grissom's trust in Sara was the reason that led him to ask her to join him in Las Vegas and entrust her with a very delicate task in the situation the lab was in at the time: after Holly Gribbs' death, to investigate what failings there had been on Warrick's part. This task is generally given institutionally to the Internal Affairs section, but Grissom decides not to involve them. So much of his personality, and even some of his refractory behavior toward Sara, can be inferred from this early behavior as the new supervisor of the forensic night shift team.

For as much as he is (or appears to be) always adherent to the rules, he feels that they can be broken in certain instances. This is especially the case when people close to him are involved/in danger: we see it with Warrick in 1x02, by his calling Sara instead of involving Internal Affairs and evidenced also by his decision not to fire him; we see it in 5x24, when during Nick's kidnapping he decides to take the ransom money to the kidnapper; we see it with Catherine in 4x15, when he sets aside the fact that she used the lab to establish a family connection of her own; finally, we see it in a more striking way in 5x13, when Sara risks being fired. The same is true in 7.23 when Grissom spends the night at Lady Heather's place. That he is able to do this only because of himself or because of the support he has among judges or other elements of the system is made more clear in episode 8x12, when District Attorney Madeline Klein tells him "If it weren't for me, you'd have no team".

Whatever the case, no matter how much Grissom denies it (to people around him and to himself), the feelings binding him to people play an important role in his life. He doesn’t know to what extent, though. Or perhaps he tries not to think about it.

And here we come to the point. Once the issue of Warrick has been solved, it is to be assumed that Grissom asks Sara to stay and continue working with Las Vegas’ team. He asked her to come to work in Las Vegas spurred by his "survival instinct": finding himself lost in a time of trouble, he clings to what he considers firm and reliable (Sara). He doesn't ask himself why he considers her "something firm and reliable," he just knows that she is. The relationship grown between them since they met has shown him that it is so. Thus, he suddenly finds this person whom he esteems and trusts (and whom, of course, he also finds attractive) moving closely around him everyday. This creates a short circuit in his mind, as Sara's continual closeness makes it increasingly clear where the roots of his trust in her go for nourishment - his heart and the love that unexpectedly sprouted the first time he met her at the Academy Conference. Since then, in a shadowy area of his mind, this feeling has continued to grow. When in episode 3x22 Sara asks him out to dinner, in fact, he says, "I don't know what to do about this": he knows very well that there is something between them, and he also knows what it is - love. In the shadows, that “something that has sprouted” time before has become a rose plant, full of beautiful fragrant flowers, but also many thorns.

The thorns protecting the beautiful flowers, the problems stopping him in the face of this dilemma, are various, but I think they could be roughly summarized in two points, which I would call: the sluice against the flood and the sheepdog instinct.

Regarding the first point, the sluice against the flood, one must consider the age difference. However much it may not be felt in certain circumstances, Sara still remains younger than him by almost twenty years. She is much younger than he is. Indeed, we see that Grissom has no problem approaching women closer to his generation - the lab technician in 1x01, Terri Miller, Lady Heather. The fact of youth is immediately brought to surface in Grissom's monologue that you mention, at the end of episode 4x12, in which he refers to "somebody young and beautiful." Grissom's tendency to want to keep everything under control, everything pigeonholed, leads him to realize the variables that come into play in establishing a relationship with someone younger, of which the main one is: what if I give in to this feeling and then she changes her mind? The same concept is presented by the culprit's words at the end of episode 9x02: "She promised me everything...and then she took it all away."

All these years Grissom has protected himself from this eventuality, keeping himself at a distance from everything and everyone. What could happen, then, if he opened the floodgates to the course of the deepest of feelings, letting it invade him, and then, instead of a nice placid stream full of little fish, the stream turned out to be a raging torrent? At work he sees it every day how destructive human passions can be. Also because of this, in episode 3x14 he says he doesn’t trust people. People are very moody, you know? Science is not. So, what would become of him if his relationship with Sara did not go as planned and she, once she knew all his weaknesses, left him? He would certainly come out of it with broken bones. As Grissom again says in the monologue at the end of 4x12, "and she showed you a wonderful life, didn't she? But then she took it away and gave it to somebody else."

By the phrase "we have to risk everything we've worked for in order to have her" is meant not just a career, but one's whole person. It refers to the work that each of us does on ourselves to improve, to mitigate or come to terms with our weaknesses, to be able to cultivate our passions, continuing to be excited by them. The work each of us do in order to become a stable person, able to stand against the winds of life. Establishing a romantic relationship with Sara, for Grissom, means laying all this bare with her. And this is risky.

So, on the one hand there is this sense of self-protection, but on the other hand the mirror of the same feeling, the sheepdog instinct, can also be brought into play: he cares about Sara, wants to protect her, and does not want to hurt her. Approaching more mature women also means, in a sense, establishing relationships where the feelings involved may be less deep. Grissom knows that with Sara, instead, there is an important feeling at stake (on both sides), so why risk giving her hopes that he may not be able to honor? In 13x15 Sara informs DB and Nick of the end of her marriage to Grissom and when asked by Nick what the reasons for the separation are, she replies, "Ask him--I'm sure he had my best interests in mind." In this regard, for the sake of clarity, let me quote a passage from the "rollercoaster fic" (I'm almost done, I'm checking the third-to-last chapter) in which I deal with this stage of their relationship:

In the first phase of their romantic relationship, Grissom had touched Sara's heart, seeing it clearly, and deduced that she was a resource that had to be protected at all costs. Finding himself alone again, therefore, he decided to close his eyes and act as if he had not attained any knowledge up to that point. Survival instinct acted for him, throwing blind fists at the air and pushing him to leave the land, to drown himself in the immensity of the ocean, keeping away anything precious that could be broken. Only in this way would she be protected.

Sara is something precious to Grissom, and we have already mentioned that he has a tendency to defend what is precious to him, be it insects or people. And this inclination is exceptionally exaggerated in regard to Sara. His stepping aside therefore is a wanting to shelter her from harm that he might do. Perhaps, he also thinks that by remaining detached from her just a little bit, he would be able to protect her better from others.

Another reason that might prevent him from throwing himself heart and soul into the relationship with Sara is the fact that the two of them are colleagues and should not establish a romantic relationship. However, we have already mentioned that when something important is involved, Grissom doesn't give a damn about the rules, so this point doesn't matter.

So what is it that ultimately makes Grissom decide to let go of his moorings aside from Sara's being persuasive?

Not one thing, not the other, but a third secret thing.

There's a passage I heard recently in the movie "In the mood for love" that I think nicely renders the feeling that moved Grissom:

Feelings can creep up just like that. l thought l was in control. But l hate to think of your husband coming home. l wish he'd stay away!

Simply put, Grissom realizes that he needs her. Yes, he told her so, but saying it is one thing, accepting it is another. At some point, he realizes that he doesn’t like she to be too far from him, he doesn’t like other men being around her too closely, that Sara is so precious to him to such an extent that he just decides to try the thing, leaving aside all the doubts and fears. The process of arriving at the complete setting of the relationship, in my opinion, took some time. Let me quote another piece of mine, “As simple as an idea”:

So he faked it. He faked it until he made it. He faked needing her for a case, when he could have easily done without her. He faked that he happened to somehow be in the same place as her by chance, in order to show her something special. He faked that she had something in her hair, in order to touch it. He faked feeling like going home, in order to convince her to do the same. He faked that a flower was evidence for a case, in order to give it to her. He faked losing his balance, in order to put an arm around her shoulders. He faked having to say something to her while sitting in the audience in a courtroom, in order to get close to her ear. He faked a bravado he didn't feel in order to kiss her. And he prayed, he prayed, that she would give him a second chance.

That’s it. I hope I was able to explain my unrefined point of view and I hope you enjoyed the typos. Keep in mind that this is just my personal opinion. For more comprehensive and technically detailed answers, you might want to check/ask the GSR-biblist @addictedtostorytelling. There you’ll surely find answers to questions you didn't even think you had.