In the episode Jerome Is the New Black" Quagmire tells why he dislikes Brian for the following reasons:
1.He constantly hits on Lois, even when Peter pays for his food and saved him from certain death.
2.He poops all over the yard as well. According to Quagmire, it adds additional insult to Peter; alongside hitting on his wife.
3.He is not trustworthy and never pays for anything, including debts and taxes; when asked to pay, he always remarks that he'll "get you later," but "later" never comes.
4.He pretends to be a deep guy who loves women for their souls when, in reality, he only dates bimbos while admitting that he does the same thing but the difference is that he is at least honest about his goals.
5.He's a terrible writer who he thinks he's great when all he does is plagiarize things off the internet.
6.He's done nothing to help despite his liberal beliefs.
7.He thinks that he's so great just because drives he a Prius. In which Quagmire says "driving a Prius doesn't make you Jesus Christ!"
8.He doesn't believe in religion and thinks it's for idiots.
9.He acts like an intellectual despite the fact that he has failed college twice.
10.He's a failure as a father and never sees his son.
11.He's a big, sad, alcoholic bore.
