This might be a case of current men's fashion creating lookalikes than an actual strong physical resemblance. The current vogue for good looking men is to have slightly unkempt hair, and a little stubble to make them look grizzled. Both Chace Crawford and Ian Somerhalder are absolutely on point with that look, and it results in one being mistaken for the other every now and then. We suppose the fact that both have a strong jawline doesn't hurt, either.
We have 'Gossip Girl' and 'Vampire Diaries' to thank for bringing both of these handsome chaps into the limelight. Somerhalder probably has the better body of work up to and including the present day, but as he's eight years Crawford's senior, he had something of a head start. Crawford, at 31, probably still hasn't hit his peak.