"It's only temporary, right?" Dean says. "Just until you're healed up, and then we'll never have to see each other again. So what do you say, Castiel, do you want to marry me or not?"
It's because Dean was an awesome brother than he took such an interest in Sam's new friend. No, really. What happened afterwards was mostly an accident.
When Castiel's offered a senior position in the Heavenly Host, he decides that before he commits himself he'd better take that holiday on Earth he's always wanted. So he does, and he meets Dean.
University AU. Dean isn't as cool as he thinks he is.
Despite their age gap and differing social circles, Castiel has struck up a warm friendship with Mary Winchester, a wealthy widowed socialite. When Castiel needs a place to stay, Mary invites him into her house, where there’s loads of spare room. Castiel’s aware that they make an odd pair, but he doesn’t fully realize how things look to outsiders, especially to Mary’s eldest son. All Dean Winchester sees is that his mom has apparently hooked up with a hot young guy (who is totally Dean’s type) and that makes things… weird.