Cas watches as Dean and Sam find John, only to lose him again in the car crash. Dean was dying, and John saved his life by giving his own. If he hadn't, Cas would have interfered, to hell with the consequences. He'd have spirited Dean somewhere, saved him, they could have lived together.
He's wanted to do it since they first met, but hasn't been brave enough. Hasn't had the free will to be able to.
Castiel doesn't see Dean and Sam fight the trickster, is caught up in other things involving his garrison. He doesn't see Gabriel, who has been missing for 17 years.
He watches Sam get stabbed in the back, but doesn't interfere. None of the angels do, not even Michael. No one helps Dean. Dean doesn't pray to Cas for him to fix things, to bring Sam back. He finds his own way. Dean knows Castiel can't interfere, surely he doesn't take it personally? Cas wishes he could help Sam, feels bad because he knows if it were Dean, he would have tried anything. Cas watches as he kisses the demon, being told he has a year left before he gets pulled to Hell. Sam comes back to life. Castiel feels regret curl in a pit in his stomach.
He sees the battle against Azazel, the opening of the Gate to Hell, John Winchester's soul returning (briefly) to help his boys, holding back the demon that killed his wife, exacting his revenge. Dean shooting Azazel square in the heart. He's proud of the Winchester, wishes he could be fighting alongside him, but daren't let John Winchester see (even if it is just the man's soul). He also doesn't want Azazel to know, because then Lucifer would find out, and that could prove disastrous.
There are so many rules and procedures that Heaven has which angels must adhere to, and Castiel doesn't understand half of them. He doesn't understand why people have to suffer, why angels can't intervene. Why destiny is the supposed removal of free will, when surely free will is what makes destiny. Being able to choose your own path is what life should be about, surely. He should be able to go and speak and be with Dean Winchester without having to worry about some greater plan.
The little angel who could.
Maybe this is his destiny. To forever be an outcast of Heaven, in love with a human.
