Damon thinks the situation is hilarious. Stefan finally got the girl. And then Elena found Katherine's picture. That was a mood killer.
Stefan's getting dressed, saying he has to look for her and he has to explain and all Damon wants to say is look at me. I've protected you and taken care of you; why do you care for her more than you do for me?! I love you more than she ever could.
But he doesn't say that. He won't say that. Because she obviously means the world to Stefan and if it makes Stefan happy, then. . .then he'll be selfless for once. He'll let his brother have this bit of happiness. So when Stefan asks him to help look for her, asks him with those dark eyes and that wounded expression on his face, Damon can't say no.
However, he won't deny that deep down, he wishes Elena would suffer the fatal illness all humans do and leave Stefan alone so Damon can have him all to himself. So he won't look too hard for Stefan's ill fated lover and maybe Stefan will see that Damon can be everything Elena can't.