Why, according to Atticus, does Bob Ewell bear a grudge?

From the text:

“I think I understand,” said Atticus. “It might be because he knows in his heart that very few people in Maycomb really believed his and Mayella’s yarns. He thought he’d be a hero, but all he got for his pain was… was, okay, we’ll convict this Negro but get back to your dump. He’s had his fling with about everybody now, so he ought to be satisfied. He’ll settle down when the weather changes.”