When someone is celebrating a birthday, there is always one person in the group who gets as drunk or drunker then the person celebrating the birthday. This is generally a result of hanging out with the birthday person the whole night and thus weaseling in on many free 'birthday drinks', while also buying their own drinks.
Bob: "Yo, did you see Steve last night .... guy was a mess"
Tom: "Yah man, he was drunker then me, and it was MY birthday"
Bob: "haha, yah man, definite case of birthday envy"
Tom: "definite!"