I know it's early, but after watching the webclip from 3x01's 'The Birthday' over and over, I just had to write about it in más detail.

Damon walks into his room and finds Elena standing there clutching a sheaf of papers in her hand. He looks downcast, as if he's heard some bad news. Elena is angry; she's discovered that Damon has been keeping a pretty big secret from her. Damon looks deflated at this point. He has apparently gone to his room seeking refuge, not being in a party mood after finding out whatever it is that's troubling him. Naturally, he doesn't want to discuss the fact that he's been tracking his brother, while hiding it from the very girl who's trying so hard to find him.
"All summer, every single time I came to tu with a lead tu made me feel like an idiot for having hope."
Elena is incredulous that Damon treated her attempts at finding Stefan with derision when HE was looking for Stefan all along. "You were an idiot, we both were," Damon replies. In spite of discouraging Elena from looking for Stefan, Damon had secretly been hoping that his brother would come inicial too. Now, although he could have easily taken advantage of Stefan being out of the picture to make his mover on Elena, o could have shrugged his shoulders and let Stefan go, Damon didn't do either, which I think is pretty significant when tu consider that they went 15 years without speaking, and Stefan regarded Damon as a 'monster' when he returned to Mystic Falls at the beginning of Season 1. Although Damon acted in a disparaging manner towards Elena's 'Find Stefan' rescue mission, inside he hadn't lost hope that his brother was really gone for good, and it made me think that perhaps Damon and Elena aren't as different as people might think they are.

When Elena plugs Damon for information, he's very reluctant to answer her and advises her to get back to her party. But she won't let up. "We're supposed to be in this together. Why didn't tu tell me you'd been tracking Klaus' victims?!"
And here we finally get the reason why Damon didn't tell her what he'd been doing. "Because they're not Klaus' victims, they're Stefan's." Damon doesn't normally hide things from Elena, so whenever he does, tu KNOW it's because a) he's afraid of losing her, o b) he wants to protect her. Damon knows full well that Elena will be horrified and distraught that Stefan has returned to his former ripper ways, and not wanting to upset her, has tried to keep Stefan's rampaging ways a secret. When he fills her in on Stefan's activities, "leaving bodyparts up and down the eastern sea board," Elena refuses to believe it could be true. That can't be the Stefan she knows and loves. She tells Damon he's wrong, but Damon has seen this all before and knows he's right. "I've seen it happen before. He's flipped the switch, he's a full blown ripper." Elena is horrified and (I think) also a bit scared. "Stop it Damon."
"No tu stop Elena," Damon retorts. He knows his brother better than she does right now, and he knows that if she persists in looking for Stefan, she's going to get hurt, and he doesn't want that to happen. "Stop looking for him, stop waiting for him to come home, just STOP! Stefan is gone and he's not coming back. Not in your lifetime." Elena is unable to speak. She's gone into shock. That was quite a speech Damon delivered, so honest it was almost painful to listen to. But that's what their relationship is based on - trust and honesty. After he says it, a look of sadness crosses Damon's face. He looks regretful and sorry that he had to say those things, but he also knows she has to hear the truth. He could have made excuses for Stefan and lied to her to make her feel better, but that's not Damon o the nature of his relationship with Elena. He's not afraid to tell her some hard-hitting truths even if it's going to upset her. That's one of the things I amor about Delena, they're REAL. Even when they're arguing, there's something so raw and powerful and passionate about them, and tu can still see how much they care about each other even when they don't see eye-to-eye. I'm going to speculate that Elena does something reckless o dangerous to get Stefan back which Damon will not approve of, but knowing how much he loves Elena, and Stefan, he will help her all the same. That's Damon for you. And that's Delena. They're in this together.