Why is it (since you and I believe Sam is the protagonist) that the show has at least 5 times given us "deep" insights into Dean, but not the same for Sam? For example, Sam took dreamroot to get into Dean's head and sees Lisa and the picnic setup, the djinn grants Dean's wish of his mother never dying, the siren gives Dean the worshipful, little brother Dean desires, Amara repays Dean for giving her what she needed by giving Dean the same and he got his heart's desire with John's return?
The short answer is the audience can’t know everything about the protagonist because if we know everything then there’s no where for the protagonist can go. They’re done. They’ve reached the end of their ability to be interesting in any way and the story is done. As in every good ending, the end to Sam’s story, and therefore Supernatural, needs to be inevitable but also unpredictable.
“Endings are a bitch because it’s very hard to find a way to both surprise and satisfy an audience.” - Renee Palyo, former ABC Movies executive and writer.
The lack of “deep insights” scenes for Sam is to prevent us from predicting his next move.  In “Prophet and Loss”, we know Sam will talk Dean out of his box plan, that was inevitable, but we didn’t predict how powerful Sam’s speech will be because he’s usually not the one to make bold speeches and declaration, or that Sam will punch Dean because that’s not what he normally does, or fall into a desperate hug because Sam doesn’t go around giving out free hugs, or turn into a lost child before our eyes because he’s been a leader and a father figure for at least three seasons.  But yet all of these unpredictable Sam’s actions are inevitable if you have been following his consistent character development for the past thirteen years. Â
Look at Star Wars Kylo Ren, we are crimininally denied deep insights on him and we wonder, what’s he really doing?  It’s the lack of insights why we didn’t predict he would fight side by side with Rey, arguely creating the most powerful and memorable scene in the Star Wars saga. Both Kylo Ren and Sam Winchester doesn’t seem to want anything at all for themselves and yet they both took the leadership role after the Mid Point Reversal of their arcs.
I feel like we know everything about Dean to the point that I get tiny bit irrationally angry whenever I see that damn pie as part of the show’s PR.  It’s old, let it go already. Give us something new and surprising about Dean.
Some of the “deep insight” Dean scenes feel like extraneous subplots that didn’t go anywhere, and it would be if it was really about Dean.  True extraneous subplots are like the unnecessary opening scene of Jupiter Ascending about Jupiter’s relatives who are illegal immigrants, this background detail never becomes relevant to Jupiter’s struggle to save Earth from a hissing Eddie Redmayne. Â
Instead the “deep insight” scenes serves Sam’s story.  Most Dean girls know the apple pie life is not Dean’s lifestyle choice and even Chuck said Dean only did it to keep his promise to Sam, but from the general audience’s view Sam made a selfless gift to Dean after he took down Lucifer and Michael because we saw Lisa and her 100 watt bulb picnic basket in Dean’s head.   The djinn and siren episode showed how much Dean misses Sam because, you know, it’s always about Sam .  Amara gives Mary to Dean and what is their most important interaction about?  Yeah, it’s Sam of course.  Dean wished for an intact family, John being the last piece of the puzzle and the episode was a powerful Sam episode.
This is why Dean is not a dual protagonist alongside Sam, nearly every “reveal” into Dean’s character ultimately serves Sam’s character; from how important he is to Dean’s life (djinn), how much he needs Sam (siren), how much he grieves for Sam’s suffering (Dean confronting Mary thanks to Amara).
The question that every storyteller ask is how do I create a captivating protagonist that readers will want to follow to the very end and live on in reader’s memory afterward, like Scarlett O’Hara, Bilbo Baggins or Harry Potter?  Scarlett was not the most sympathetic character and Bilbo and Harry were not the most interesting characters at first glance, but readers were nonetheless compelled to follow their journey AND remember them afterward because we couldn’t predict how they would react to the world changing around them.
