The Things That Dean Doesn't Know [Tessa gave him his memories back from the post-crash stuff, though. Also see: Dean doesn't know that Sam spit demon blood back out when demons tried to force him to drink it.
Sam loves Dean, and Dean is super overprotective of Sam because he loves his brother right back :) it's what family does. So if anyone says that Sam doesn't love dean as much as dean loves Sam just read this post
The bond these two have is incredible. Despite the fact that Sam desperately wanted to distance himself from all of it, he and Dean are so alike in the way they think, it's scary.<<< they probably thought up a plan before hand in case this ever happend
I just really hope Dean and Sam would still be close if Jess didn't die, because that's why they became brothers again; Jess died and Sam wanted revenge, and along the way, he realized what an awesome brother Dean is. It hurts so much!
If Cas learned this kind of love from Jimmy [powerful family love] it makes sense. And I personally think Dean taught him a different love, unintentional on Dean's part. I mean Cas cared deeply about his angelic family too.