it's literally impossible to put down in a simple thread what they mean to each other, every soft touch, concerned look filled with care, every hug and simple gesture that speaks of love and respect... but let's begin
their story began when castiel, the angel of the lord loyal to the heaven, rescued dean from hell. however, despite all the obstacles and even god himself, this is not where their story ended
cas was a warrior, older than the world itself, but only a few weeks of knowing dean winchester were enough for him to began questioning heaven. in this scene, he opened up to dean for the first time, voicing his doubts in the orders he was given
ofcors, eventually other angels noticed cas was not the same, they've noticed that cas became too close to humans in his charge, to dean, so they tortured him in heaven hoping they will regain control over him
but we all know how that worked out, don't we? in the end, cas choose to rebel the heaven, he choose dean
so, what did that choice meant for cas, and what did that choice meant for dean? to simply put it in a few words, it meant everything. it led them to 10 years of laughter, tears, fights, deaths, revivals and everything in between.
so let's remember that time dean told cas to never change, or ALL of those times cas had a problem with personal space
or the time cas straight out admited he and dean do share a more profound bound. all of those little moments are a part of a bigger sum, a part of their past, present and the future.
the matter of the fact is, even when one of them was gone the other just couldn't simply move on. i mean, dean literally carried cas' trenchcoat everywhere with him for months until cas came back
and let's please not forget purgatory and how dean did everything he could to find cas and flat out refused to leave it without him
also, the testament of how strong their bond actually is, is the fact that even naomi's brainwashing of cas was broken by a simple "i need you" by dean
the look on dean's face when he thought cas was dead (again) tells you more then i ever could... also the way he touched cas' face....... but let's get back to the story
so we established that cas rebelled heaven for dean before, but that wasn't the only time he choose dean over his family (angels). he gave up his entire army over dean in s9. even metatron saw right through him and said that everything cas have done was about saving dean
i mean, cas was even willing to go with dean on suicidal mission to stop amara just so dean wouldn't die alone
though what often get's overlooked is that cas is to some extent dean's weakness as well. if you think about it, after cas' death in season 12 dean was not the same, he was depressed, defeated and he wanted to die
in this 11 years they've been through a lot, just think about how many times they've lost one another, whether it was death or something else that kept them appart but also think about how many times they've managed to find their way back to each other