Because doofcas wants Cas as a trapeze dancer, and I concur.
Sometimes, Dean’s heart is in his throat.
Sure, his tricks are dangerous; throwing knives isn’t exactly safe, but it’s nothing like what Cas does. Nothing like dancing who-knows-how many feet in the air, without any harness or net to catch you. Hell, Cas doesn’t even have a partner to catch him; not that that would necessarily help if something went wrong, but it just feels safer out there when someone’s got your back.
Take Dean, for example: Dean has Sam.
Cas, Dean’s best friend, was dropped by his troupe around the time they became, and because, they became best friends (“You’re getting too close to the competition, Castiel”). So, not only does Cas not have a partner, but he doesn’t have a partner because of Dean. (Cas, of course, vehemently denies this. He’s a damn liar.)
Admittedly, Dean had tried trapeze, so he could be Cas’s partner, but it turns out that freezing up when you’re swinging fifty feet in the air isn’t actually how you do it. See, Dean’s a little bit afraid of heights.
Dean’s also a little bit in love with Cas, though, so the terror of being so high up for such a short period of time had been pretty much counterbalanced by the fact that Cas had taken care of him afterward. He’d smiled a lot. It’d been pretty great.
…Dean is maybe a little bit more than a little bit in love with Cas. From the very first time Dean had seen Cas perform, he’d pretty much been a goner.