AnimeXen

He goes from completely exaggerated facial expressions and gestures: 

When I get a scene with Astarion, I always reload a thousand times to try out all the routes and see all his reactions, so what I'm about to break down will be no less than heartbreaking.

Why does Astarion fake confess his love to Tav?

First let's take a look at what Astarion has to say. He starts the conversation with a clear goal in mind: he wants to seduce us again and he says so right away. He continues with very openly displaying his skills at charming people with honeyed words, seemingly enjoying himself while doing so. But what we have to keep in mind is that he's done that thousands and thousands of times already. I think it's safe to assume that he's perfected this mask of a smug, flirtatious man enjoying all this debauchery. 

But after giving us one suggestive line after another, he does something that does not match the tone of the conversation so far: his fake love confession. I mean, just look at him. 

When I'm with you, I feel practically alive, yet I crave only to die again with you.

To this:

I love you

We get a closeup of his face and see that he gets serious all of a sudden despite the fact that the conversation has been all fun and flirty just a moment ago. He draws his brows together, tells Tav "I love you" and then just looks at us with this serious and almost sad expression. Completely different from before when he was just toying around. 

And yes, I just said he's very practiced in playing pretend, but just the difference between shallow and cheesy lines about craving to die together and their perfect body whispering temptation in contrast to a simple "I love you" and then on top of that the difference on how he conveys it - for me it just doesn't fit together, this isn’t Astarion pretending. And furthermore he gains nothing from adding this confession. The flirty lines would have been enough to seduce Tav once again and therefore ensure their attachment to him, so that his protection is granted. And that is all he wants, at least in the beginning. 

But then, why would he voice this fake confession in the first place? And that's where my theory sets in. I don't think he would risk the trouble of getting more feelings involved than necessary in his "nice simple plan" - seducing Tav, sleeping with them, manipulating their feelings. For doing so, he simply wouldn't need a love confession. 

With saying "How about I say those little words. Everyone's favourite" he already suggests that he's said them to a lot of people already. And this surely is a way to charm and seduce people very fast, but with Tav he doesn't need to rush. They're tied together for an indefinite amount of time, he doesn't need to seduce them in a matter of hours or days before serving them to his master. And he's already seduced Tav successfully before, so my guess is this:

My explanation for the fake confession

But what if he started to fall in love with Tav even before the fake confession? Think about it, he has slept with Tav once - successfully seduced - so all he has to do is sustain this kind of relationship. Saying all his favourite lines at Tav and trying to get them to sleep with each other for a second time would have been just that - sustaining the relationship at present. But then he says "I love you". And I say he does so purposefully. It's a test. Having someone like you enough to sleep with you is one thing. Having deep and complex feelings like love involved is something else entirely. So he tests the waters with this confession. He hides behind all of these flirtatious lines and places the confession at their end to throw a veil over its real meaning. 

The real confession scene, where he admits to having started to genuinely feel something for Tav, came a bit out of nowhere for me. (And don't get me started with our options to react to his real confession. I talked about that here.) I don’t think he would just willingly tell Tav that he's grown fond of them, when a few nights before he threw the fake confession their way without giving a damn about Tav's feelings - that just felt very out of character for him in my opinion. 

It may be a lie either way - saying it just for entertainment or saying it to see if Tav is open for deeper feelings - but I think he exaggerates on purpose to trigger a reaction that speaks for itself. Does Tav care about these words? Is Tav hurt to see that Astarion is apparently willing to play with their feelings like this? Or do they not care about such vanities? 

I want to emphasise the fact that Astarion hasn't had the luxury to allow himself anything resembling these kinds of feelings for at least two centuries. Naturally, he would not just come around and open up his heart to Tav when all he's done until then was charming them just for his plan. He would want to reassure himself that this wouldn't turn against him if he dares to show such great vulnerability in front of anyone. He says it himself when he properly confesses that it is intimidating for him to make decisions on his own again. And confessing his feelings is exactly one of those intimidating decisions.  

Interpreting Astarion's reaction

To back up my take I want to take a closer look at Astarion's reaction when we reject him after the fake confession. After he laughs the confession off in one way or the other, he insists on having sex with you again. Here he sticks to exaggerated facial expressions and gestures, just like before with his honeyed words.

Now, as much as I relish standing around and saying all my favourite lines at you, I’d much rather we got to experience each others’ full portfolio of talents once again.

But if Tav then turns him down with saying “I don’t think I really want this”, this is his immediate reaction:

Well, well, excuse me while I die of a broken heart.

He takes a few seconds to process what Tav said, and then instantly responds with another exaggeration accompanied by fitting gesticulation, ending with a fake smile. After that he just looks at Tav for another few seconds and his demeanor changes again:

In all honesty, it's a shame. That time was special to me. I've gotten on my back ten thousand times or more, and forgotten half of them. 

He loses his fake smile, averts his gaze and starts fumbling with his hands nervously. He can't hide his disappointment, but he tries to keep his face neutral - even if it doesn't work all too well. He tells us we were special before giving us a scrunched fake smile once again. It's really hard to watch…

But you… you I'll remember. 

For usually being rather quick to react and respond, we can see how difficult it is for him to react to Tav in this case.

Have a fine evening, dear. 

He pauses yet again, lifts his gaze to the sky and just looks so lost. Mere moments ago he was flirting and laughing, and now he seems so utterly desperate. And then, he can't even keep up his attempts of masking his sadness when his expression slips in the end and his face scrunches painfully for just a split second before the whole scene ends. This really broke my heart.

Conclusion

This is definitely not the reaction of someone who was turned down for just another round of sex - even if this meant that Astarion's plan of assuring his safety didn't work out. Because then he could have just tried to get Tav to change their mind with another one of his favourite lines. Or he would have at least reacted like he does when you tell him to stay at the camp ("Oh darling, I'm hurt" which he obviously isn't). But he doesn't. 

Because this is the reaction of someone who dared to get his hopes up. Someone who thought that maybe, just maybe, these unwanted, complicated feelings for Tav which had slowly crept up, nullifying his nice simple plan, could genuinely lead to a relationship he didn't know he needed so desperately. Someone who is so devastated by a rejection at this point, that he doesn't even question it. He just accepts that Tav isn't interested in sleeping with him, let alone having feelings beyond sexual desire. It's just as it always has been. It's not as if Astarion remembers anyone caring about him, so why should it be different this time… 

And that's why I think this whole fake love confession was a way for Astarion to veil his growing feelings for Tav under the pretence of flirting, and had the purpose to find out if he could more or less safely confess his true feelings to Tav.