Castiel knows this is a punishment, but for what, he doesn't know; he's done everything Father's ordered. He's always obeyed(but not with Dean Winchester; that's why he's here-) Uriel had chastised him, warned him that he was getting too close, but Castiel couldn't keep his heart from drifting(But God knows he tried and yet Father doesn't seem to care as his brother sears the lesson into his flesh-).
Dean tempts him with rousing speeches about God's lies and pie and family(he'd do anything for his brother; Castiel would give anything to be that for Dean, anything for Dean to rescue him, but he did; he rescued him from Alistair-) Dean tempts him with every "Cas"(he had "googled" nicknames and learned that nicknames are often used affectionately in this society; the thought of Dean thinking of him affectionately was enough to make his heart flutter).
In his time away, his wings are ripped out only to grow back; they whisper cruel things in his ears about falling and listening to orders(they told him Dean thought he was a monster, that Dean wouldn't miss him) and they tell him they won't send him back unless he agrees to shut the doors(feelings, doorways to doubt); they won't let him see Dean until he agrees and when he can't stand it anymore, he closes his eyes and sees Dean's cocky smile and agrees with a choked sob.
It had been Father's order to rescue Dean from perdition, to help him save the world; Castiel remembers that when Dean asks him, remembers it when he replies "I learned my lesson while I was away, Dean. I serve Heaven. I don't serve man. And I certainly don't serve you." It had been Father's order to stay with the Winchesters, to stay with Dean, but he can't understand why Father would do this to him.(Lead me not into temptation; that was the Lord's prayer, but why would Father lead him into sin, into unbearable temptation? Why would God lead him to Dean Winchester only to be denied what he cherishes most?)
When Zachariah traps Dean in the green room, Dean begs him to help him, to help Sam(but all Castiel cares about is Dean) and he's so close to being tempted again, against his orders, so he lashes out, talking about pain and he makes sure it's about Dean and not himself; he offers Dean Paradise and redemption, makes sure to include Sam(because Dean would never accept Paradise without his brother), but Dean throws it away and tells him they're done(they can't be done; they never began).
Castiel, in his defense and much to his pride, lasted about two hours before he was back, pressing Dean against the wall(for a moment, he's tempted to know what a kiss feels like, tempted to know what Anna felt when she kissed Dean, when she and Dean-), but he ignores it and slits his forearm to provide blood for the seal; it repels Zachariah and gives them enough time to reach the prophet's house.
He remembers Dean's surprise when he tells Chuck "Well, we're making it up as we go", remembers when the archangels arrive, remembers thinking this is more punishment; he knows he hasn't learned his lesson when he sends Dean away(presumably to stop the apocalypse, but in actuality, it's to spare Dean the archangels' wrath; he won't have Dean punished for his sins). Chuck lays a hand on his shoulder, but one look from Castiel makes that hand withdraw(he doesn't want Chuck; he wants Dean; that's all he's ever wanted, all he's ever allowed himself to want-why did God do this to him?)
But then Castiel hears a whispered Cas and he looks towards the coming archangels with a smile because he knows that any punishment was worth it just to hear Dean say his name, to see Dean smile at him. Chuck asks him "Are you crying?" as the light pours over him, Castiel realizes he didn't know how to cry before he met Dean.