"New findings published by the University of Michigan say that men prefer women who were dumped by their last boyfriends, while women prefer men who initiated their last breakup. ... [The scientist] speculate that when a man initiates a breakup, his actions reinforce the male dominance that is inherent to gender roles. Therefore, knowing his prospective girlfriend was dumped reassures him that he'll be the one in control. On the other hand, when women leave their boyfriends, guys may consider them high-maintenance and picky, which could also make them worry that she will eventually reject him as well. Meanwhile, women may prefer guys who initiated their last breakup because it suggests that he isn't hung up on his ex."
This reinforces some very sexist ideas, but I can certainly see why women prefer guys who aren't hung up on their exes. I would also prefer a guy who initiated his last breakup. But I think it speaks to a lot of insecurity in guys that they prefer women who have been dumped!