Answer:
Oskar's grandfather leaves his grandmother because of the rules.
Step-by-step explanation:
in Extremely Loud and Incredibly Close, Oskar's grandfather leaves his grandmother because of the rules. The rules between them were supposed to help Oskar's grandpa feel like he had more control over his life. Yet soon the rules got out of control. He realized his marriage wasn't based on feelings, but rules. The rules were separating them, which is why he separated himself from his wife.