The wildly popular and long-running series the show's protagonists. The two have been known to form unlikely friendships and allies in the strangest of places, but sometimes these friendships and alliances are too weird, even for Supernatural.
Dean and Crowley's relationship is one such example. For a human and a demon, the two got along on surprisingly well , which just doesn't make any sense whatsoever.
Dean Kept Crowley Captive
Alongside Sam, Dean kept Crowley captive at the beginning of Season 9 in their bunker. Crowley doesn't like taking orders, much less being forced to do anything. He did strike a deal with Dean to be freed, so long as he helped Sam toss out the angel that was possessing him, and Crowley did hold up that end of the deal.
However, Crowley didn't appear to hold too much of a grudge against the Winchesters after being their prisoner for so long, which was no doubt frustrating and humiliating for the King of Hell. He did go back to taking over Hell and went about his business after he was freed, so perhaps he was too busy to hold such a grudge.
Crowley Killed The People The Winchesters Saved
The Winchesters usually don't forgive---or forget. There was a time where Crowley went after the people Dean and Sam had previously saved from other monsters, trying to get the boys to give him what he wanted. He even killed Sarah Blake, the first woman Sam seemed to have feelings for after losing Jessica in Season 1.
Not only does it undo their hard work, but Crowley also went after people who'd already been through and seen horrible things that had nothing to do with what he wanted from the Winchesters. It's punishing the Winchesters in the worst way; how could Dean even stand to look at Crowley after that?
They Insult Each Other
For the majority of the time, Dean and Crowley toss insults and never miss a chance to degrade the other. Their relationship is mostly built on a mutual goal, like taking out the Leviathans, and on top of that, their witty yet insulting banter.
It's something audiences love to watch, but for Dean and Crowley, wouldn't that grow old? Wouldn't they be more annoyed than not with one another? They're not around one another all the time, but when they are, it seems they'd grow sick of one another quite quickly.
Dean Hates Demons
Even the word 'hate' is an understatement given how Dean feels about them. He hates dealing with them, from their trickery to their mockery to everything in-between. He doesn't trust them and clearly has a low opinion of them. So why does Dean tolerate Crowley most of the time? Crowley isn't just any demon either---he's the King of Hell.
Audiences would think that Crowley would be at the top of Dean's blacklist, and there are times when it appears that way, but Dean and Sam use Crowley as an ally more often than not. Granted, the King of Hell is good to have on their side, but when you consider how much the Winchesters, especially Dean, hate demons, how would that even work?
There Is A Mutual Distrust
Dean knows not to trust Crowley; he knows the King of Hell well enough to know he'll turn on anyone, anytime. Crowley seems to share in that distrust. They trust each other enough to do what needs to be done, but they never trust one another completely.
It's a wonder their relationship works the way it does when the trust they show one another is limited in nature, and never seems to grow all that much. Both Dean and Crowley take the saying "keep your friends close and your enemies closer" to heart, especially in this instance.
Dean Tried To Turn Crowley Human
On the flip side of the coin, Crowley detests humanity as much as Dean hates demons. The Winchesters were trying to cure Crowley of being a demon for the purpose of forever closing the Gates of Hell. Crowley definitely wasn't a willing participant.
Trying to turn Crowley human against his will is quite brutal, actually. Keeping him prisoner (which they do afterward) is another thing entirely, but trying to change Crowley, even with good intentions in mind, would surely make Crowley furious. In true Crowley form, he's sarcastic and unwilling to help the Winchesters with anything at all, so that was obviously his way of punishing them for trying to change him.
Crowley Tricked Dean Big Time
Crowley and Dean searched for the First Blade together. Crowley ultimately swindled Dean into getting the Mark of Cain, knowing it was a requirement to utilize the First Blade. This landed Dean in all kinds of trouble and ultimately, Sam's efforts to remove the dangerous mark unleashed the Darkness.
Crowley never showed remorse, and Dean was definitely furious about having to deal with it and the consequences that came with it. This deceit definitely didn't score Crowley any points with Dean.
They Use One Another
Whether it's for saving the world, trying to get further in the world, or something along those lines, Dean and Crowley aren't afraid to use each other to get what they want. Crowley has a long history of making a means to an end in his own best interests, usually having something to do with his power over Hell and furthering his own status.
Dean has his reasons too, more often than not using others for their skills or connections to help save the Earth or Sam. It's not the most meaningful of relationships considering Dean and Crowley just use, and often manipulate, one another, but it seems to work for them.
The Time Crowley Snatched Bobby's Soul
Crowley promised to promptly return Bobby's soul when Bobby exchanged it for Death's location, but Crowley being Crowley, went back on that deal. And he even documented the kiss that sealed said deal, much to Bobby's embarrassment. It took a while, but Bobby found a way to get out of his deal, and with Sam's and Dean's help, ensured that Crowley would be forced to break their deal, given Sam and Dean were in Scotland ready to burn Crowley's bones, thus ending his existence once and for all.
Now, considering Dean and Sam see Bobby as a father, and Crowley's existence was threatened, audiences would think there'd be a mutual hatred between the two parties for this entire incident; however, they move past the incident sooner than would be expected.
When Crowley And Dean Were Besties
When Dean became a demon, he and Crowley were attached at the hip. Nothing about this newfound friendship made sense. Dean seemed to be the leader; why would Crowley put up with that? Not to mention, why didn't Crowley take better advantage of Dean's demon status and wreak havoc with him? It's what was expected from Crowley's character.
Also, why would Dean hang out with Crowley of all people? It seems more likely Dean would've gone solo as a demon. This period of friendship, while amusing, diminished both characters in the sense that neither were really themselves while playing out this storyline, making it far from realistic.