In T. C. Boyle’s Greasy Lake, why does the narrator feel like crying when “the girl” offers to party with him and his friends at the end of the story?

The narrator feels like crying because he suddenly realizes that a decadent lifestyle often comes with shocking consequences. Although he might have accepted the girl's invitation in the distant past, his recent encounter with Bobbie, the "greasy" character, has changed his perspective on life.
At the beginning of the story, the unnamed narrator tells us about the excitement inherent in being "bad":

There was a time when courtesy and winning ways went out of style, when it was good to be bad, when you cultivated decadence like a taste.

The narrator's initial obsession with being the sort of "bad" character sought after by pretty girls is shared by his friends, Digby and Jeff. However, the trio's encounter with Bobbie changes their collective perspectives.  
Accordingly, the three friends mistake the "greasy" Bobbie for a friend of theirs. They proceed to harass Bobbie and his girlfriend as the couple make out in the car. When the furious Bobbie emerges from the car, however, the friends suddenly realize their terrible mistake.
Bobbie comes at them, and the boys take turns attacking him. In the end, the narrator uses a tire iron to bash Bobbie in the head. Bobbie falls, and the general assumption is that he is dead.
Later, the boys discover how wrong they are. Bobbie (very much alive) returns with two thug friends, and they begin demolishing the narrator's car (which actually belongs to his mother). Bobbie takes the tire iron to the car, while the other two use "tree branches and skull-sized boulders" to smash the abandoned vehicle. Before they leave, the three thugs throw refuse, bottles, rocks, dirt, and even used condoms into the driver's side window.
Meanwhile, the narrator's increasing distress is matched only by his terror at finding himself lodged next to a dead, rotting body in his watery hiding place. He imagines that the dead man met his untimely death because of a bad drug deal.
So, by the time the girl invites the narrator and his two friends to party with her (and her friend), the narrator has had enough. He's lost his taste for being a "bad character" and almost cries at the thought of it. The narrator has come to realize that thugs like Bobbie really do exist and that "bad characters" sometimes end up dead in a greasy lake in the middle of nowhere.