Simpkin dislikes Alan and John. Why?

Simpkin the miller previously cheated the college that John and Alan attend by overcharging them for the flour he grinds in his mill. They come and try to trick him by claiming that they're interested in milling; they want to watch him grind it, in reality, so that they can make sure he's not charging too much. So he doesn't like them at first because they are trying to prevent him from making as much money as he has in the past and also because they are treating him like he's stupid by trying to play this trick.
Simpkin responds by stealing the grain and having his wife bake it into a loaf of bread for his family. He also unties the students' horse so that they have to leave and chase it down while the stealing and baking happens. When they return, they ask to pay him to stay there overnight, and he agrees. Their plan is to get revenge against the miller for untying their horse and stealing their grain. To do so, the students have sex with the miller's wife and daughter. Understandably, Simpkin hates the students even more after this, so he attacks them. In the fight, his wife ends up injuring him by accident instead of one of them. The students beat him up, take the loaf of bread and their horse, and run away.
The Reeve's Tale has to have an unhappy ending for the miller character because the Reeve tells it to make fun of the Miller. The drunken Miller has just told a tale that made a carpenter—the Reeve's former profession—look bad. So the Reeve gets back at him by telling this story, which doesn't end well for Simpkin.