Why does Glen Quagmire hate Brian Griffin in Family Guy?

I've been noticing that in the new episodes Quagmire is starting to get mad at Brian for very small stuff and in one episode Brian and him had dinner and Glen told him exactly why he hated him but these are reasons that he barley cared about. Also, in the episode I watched last night Quagmire and Brian apologized for dating each others ex girlfriends and at the end Brian asked him to drive him back to Peter's house but Glen smiled and ran him over instead. In the old episodes they were really good friends but why does Quagmire hate Brian all of the sudden now?