It's rude to skip a good friend's party.
Zack promised to go to his friend Slater's birthday celebration, but has a date with his girlfriend Kelly on the same night.
Zack wants to make a choice.
Zack shows up to Slater's house with Kelly.
Zack never show's up to Slater's celebration, instead going to the movies with Kelly.
Slater is happy to have more people at his event and enjoys hanging out with both Zack and Kelly.
Slater's feelings are hurt because Zack didn't keep his word.