A song that kept playing over in my head after seeing clips of 2x12 was this one. (It's been sung by many people, but I prefer the one performed by Adele.)

"When the rain is blowing in your face
And the whole world is on your case
I could offer you a warm embrace
To make you feel my love"

"She inexorably always, at the end of the day, feels for him and expresses that feeling and sort of love, in a way."

Ian Somerhalder on DE in 'The Descent'

I was kind of shocked to see people commenting on YouTube that the whole reason why Damon went off the deep end was because of Elena, claiming that if it wasn't for what she said to him, he would never have reverted to his former people-biting ways. I'm not sure if that's correct. I don't think you can hold Elena responsible for Damon's actions, simply because she encouraged him not to give up on his humanity when he was so close to it. What WAS she supposed to say? "I'm your friend, and a friend usually knows when their friend is hurting...but because you'll never say how you really feel, then there's no point in trying to get you to talk." She knows his "Death comes to us all/I'm just glad it's over" speech is nothing more than bravado he's putting on to disguise how upset he is. I thought it showed how well she knew him, and most importantly, wasn't about to let it go because she believed in him. She comes back to see him even though he tells her to leave, and she hugs him after he's thoroughly angry, defiant, cold and bitter with her. It's only then that the mask he's wearing begins to crack, and you can see that he's really struggling to hold back all the hurt which is dangerously close to breaking the surface. Damon is emotionally drained after compelling Rose's dream and staking her, not to mention the fact that he feels guilty for her fate when he was Jules' original target. The incident on the road would have happened with or without his exchange with Elena, if not that night, then later on. I don't think viewers are justified in blaming Elena when Damon was a mess anyway.

"You can never forget it. What it's like to be human. It haunts me. It's the only thing that's kept me going."

I think one of the purposes of the story with Rose was to give the audience an insight into the vampire/human perspective, which had previously remained unknown, especially when it came to Damon. Rose reveals that one of the worst parts about being a vampire is the need to kill, and the pain and regret it causes her afterwards. I think all the things she says strikes a chord in Damon even though he's loath to admit it.
I didn't particularly care for the Damon/Rose relationship. I could never agree with those who believe he loved her, but they connected deeply as friends, not as vampires but on a very human level. Her death took him to a very vulnerable place, which is exacerbated by loving Elena; and the knowledge that as long as he is the way he is, he can never deserve her and can never be with her.

"That would be human of me, wouldn't it, and I'm NOT human."

One of the slight issues I had when reading the 'Twilight' saga was that I thought Stephenie Meyer made being a vampire sound better than being a human. (Who wouldn't want to be eternally young, beautiful, strong and powerful when you could control your thirst for blood with relative ease?) The show has made it very clear that Stefan sees his identity as a vampire as a curse, but until now, we were never quite sure how Damon felt about it. While it was pretty obvious that Stefan would trade in his vampire existence to be human in an instant, I was always inclined to believe that Damon didn't really care. In Season One, he constantly ridicules his brother for drinking animal blood, and taunts him for trying to be a 'normal' teenager. I thought that Damon seemed the most OK with being a vampire out of all the 'good' vampires on the show, including the Cullen brood in 'Twilight'; and that he wouldn't go back to being a human even if he was offered the chance. All this makes his secret even more of a revelation and even more heart-breaking.

"I have a secret. I have a big one. And I've never said it out-loud. I mean, what's the point? It's not gonna change anything or make me good."

Damon doesn't feel worthy of Elena, and even when she encourages him not to give up, his feelings serve as a constant reminder that although he feels the same things humans do, he's not the same as them, and he never will be. Perhaps it's beginning to dawn on him that being the way he is, he can never truly deserve her. If he doesn't deserve her, how will she ever grow to love him? His instincts to kill combined with his self-destructive tendencies means that he's fighting a losing battle. Instead of living in pain by trying to embrace his humanity, he chooses to reject it. Perhaps it's not what he ultimately wants, but he believes it's the easiest way.
We don't know how long Damon has missed being human. Did he miss it when he was obsessed with Katherine? Because his love for her didn't exactly bring out the best in him, did it? Was it something that came on gradually? Did his friendship with Rose play a part? Was it loving Elena that made him realise how much he missed it? We just don't know yet. But here are some things about the episode I would like to point out:

In spite of brushing off Rose's demise as an everyday occurrence, Damon gives her a dream about a time when she was human in a place which no longer exists, thereby giving her the most peaceful death he can.

When anyone else would be stung by Damon's coldness and indifference, Elena isn't, and shows she cares about him by returning to the Salvatore mansion to see how he is, and hugging him goodnight.

Damon is tormented because he's not human: "I can't be what other people want me to be. What SHE wants me to be." You get the impression that he would opt to become human again if that's what it took for Elena to love him.

I think the love triangle/love story is much more than a girl trying to choose between two loves. After 'The Descent', I began to feel that it's about how far you would go for love; what you would give up, and what you would do for it, even if in Damon's case, it means going back to the dark side because living without it is just too painful. While some people might write DE off as a lost cause because Elena can't love him as he is, I don't see it that way. I think that her feelings for him run too deep for her to walk away from him, even in times of darkness as shown in this episode here. As for Damon, well, we may have seen him return to his former dark ways, but I highly doubt that we'll see the end of his feelings for Elena. Love is one emotion you simply can't turn on and off at will.

"I could make you happy,
Make your dreams come true
Nothing that I wouldn't do
Go to the ends of the earth for you
To make you feel my love"

If this doesn't make you think of everything Damon's done for Elena, then I don't know what does.