If I was forced to answer that question directly and succinctly, I would say that it's rules that caused Oskar's grandpa to leave his grandma.

As you know from having read this peculiar and captivating book, Oskar's grandma and grandpa made lots of rules. They had a rule against listening to certain music. They had a rule about never watching TV shows concerning sick children. They also had a rule about never sleeping more than once in the same bed.

"So many rules," says Oskar's grandpa, "sometimes I can't remember what's a rule and what isn't." He goes on to say: "I'm leaving her today, is that the rule we've been organizing ourselves around this whole time, or am I about to break the organizing rule?"

The rules seem to try and give Oskar some semblance of control over his life. After having survived the horrible bombing of Dresden during World War II, we might think of the rules as a way for Oskar's grandpa to regain boundaries and a feeling of command.

Yet as Oskar's grandpa admits, the rules "didn't work." He confesses that what brought him and his grandma together wasn't feelings or love, but rules. It was "a marriage of millimeters, of rules."

We'd argue that the rules that brought them together were the rules that separated them.

Although, perhaps we shouldn't be so confident about our "rules" answer. Perhaps we'd be wiser to listen to Oskar's grandma. When Oskar asks her why grandpa left, she replies, "I don't know."

Maybe "I don't know" is the more apt answer. Maybe you could argue that Oskar's grandpa doesn't truly know why he's leaving. He thinks it might be a rule for him to leave her. He also thinks he might be breaking a rule if he leaves. His uncertainty matches the uncertainty of the grandma.

Alas, maybe "rules" is the wrong answer. Maybe it's "uncertainty" that causes Oskar's grandpa to leave.