Dean:
He always knows what you need. You can be sitting with the computer, researching, wishing you had a pen to write something important down and he will hand you a pen knowing you prefer it. Or you can be eating food togheter and he will hand you the salt before you even can ask because he knows you like it salty.
Sam:
His smile. No matter how bad things get Sam will always smile for you. He'll be goofy and funny, just for you. His smile is so warm and loving that you feel safe even if it's just for a little while.
Castiel:
His innocence. You think it's adorable how he doesn't understand everyday human things. How Dean's dirty jokes always goes right over his head and how he, whenever he doesn't understand something, turns to you for advice.