After the Fall, aside from the shocking pain, Castiel found that very little had changed. So ok, he had to eat and then there were certain associated issues to work out, but beyond that he found that nothing else felt any different. He still didn't understand the more subtle human interactions, he still couldn't tell a joke and he still felt the crushing loss of his Faith. The last he'd honestly hoped would burn away, as his Grace had.
There was one change he noticed though. Sleep. Now he needed sleep. The first few nights were a blur of pain. More dazed than asleep he lay in a field, naked as a babe, shivering in pain and shock. He was found, cared for and, after the fourth day of his humanity, Castiel finally slept. True, deep, dreamless sleep.
On the fifth day, Castiel dreams. He has no memory of it, only a vague sense of warmth and comfort which fades on waking. He dismisses it, there are larger problems. He must find Dean Winchester, this he remembers. Sam has said yes to Lucifer, Hell is rising and Dean will need his help. No longer an Angel, he will be of little use in his current state, he knows this. He will offer his assistance anyway.
By the end of his first year he has found Dean. He has not yet remembered a dream. In recent months he has found himself plagued by nightmares, in these he remembers being an Angel. He drinks to forget them and is becoming less and less effective in the fight against Lucifer. He is chosen for fewer missions, often not leaving the compound he helped build for weeks at a time. The nightmares get worse and he turns to stronger and stronger chemicals to numb the terror of his humanity.
By the second year he is an addict. Perpetually removed from reality by a haze of chemicals, he watches life in the camp pass by. He remembers little of this time.
He doesn't know what changes this. Gradually though, he comes a little closer to the world. Dean allows him to help on supply runs again. Castiel is worried by Dean. Somewhere along the line, when Castiel wasn't watching, Dean changed. This Dean is hard, cold, calculating. He tortured. When Castiel brings this up Dean hits him. Castiel spends a week high as a kite.
Then Dean from the past turns up. Castiel is both amused and intrigued. This man is still young. Still has hopes. Still thinks he can save the world without losing his brother. Castiel knows this not to be true. He has seen Sam's choice. He has lived through the destruction that followed. The present Dean knows this too, has lived this too. Castiel listens as present Dean outlines his plan. His plan to kill the Devil and all that remains of Sam. Of course he will accompany present Dean. They leave in three hours.
Castiel sleeps.
Castiel remembers warmth. He remembers light and the sound of music playing from an old stereo. The feel of leather against his hand is more real to him now than the last five years of blood and pain, more real than his Fall. He dreams of hamburgers and of confusion as a young man laughs breathlessly outside a brothel. The smell of motor oil and grease fills his nose and in his dream Castiel smiles.
He is still smiling as he awakens, to a world of horror and torment. He is still smiling when he climbs into a truck with the younger Dean and drives off to help kill the Devil. Even as he dies, Castiel still has a smile on his face.
On the day he died, Castiel finally found what it is to be human. Castiel dreamed of hope.