There's plenty left on the Pop-Pop Tree, Frank!
|—Stan mocking Frank's smaller income|
Stan openly hates Frank and vice versa as Frank sees Stan as "been after him since day one" because of the fact that he got Sue pregnant before she finished college and he also believes Frank can't support her or their kids openly mocking Frank's smaller income compared to his own. Stan just wanted what was best for his 'little girl' and felt that Frank getting her pregnant before she finished her college ruined her chances of becoming successful as he is. If Sue hadn't got pregnant before she finished college it is likely Stan and Frank's relationship wouldn't be horrible as it currently is. Because Stan can’t forgive his son-in-law, he is currently one of the characters that is responsible for Frank‘s arrogant nature along with Dunbarton and many others.
In "The "B" Word", Stan revealed that his wife wasn't actually his first love. When he was 17, he had a girlfriend named Sophie he loved very much, though his father, a police officer, didn't approve of this. As a result, Stan's father framed Sophie for a crime by planting stolen evidence in her bag, which led to Sophie's father hitting her down a flight of stairs and she died. In the end, Stan believed his father did the right thing "looking out for him" like that as he nearly threw away his life for an infatuation. He thinks about this all the time and wished he did the same thing to Sue so that she'd never met Frank. Of course, this just makes Frank hate Stan even more than he already does.