George was hysterical when he's obsessed with how Jerry's girlfriend dislikes him for his strange behavior. It was pathetic and funny while watching him with his attempts to get her to like him (This woman hates me so much, I think I'm attracted to her).
Another great moment about this episode was how angry George's girlfriend was getting because she was getting fed up with him talking about his problem. Eventually, she dumps the poor fool.
This is why George Costanza is my most favorite Seinfeld character. He's a loser, nobody really likes him, even Kramer doesn't want anything to do with him. He never has anything going for him. George has a big mouth, doesn't care what he says, and he has no idea when to shut up. He's self-centered, doesn't think about other peoples' problems.