We were incompatible in a lot of ways. Like for example, I was a night person, and he didn't like me.

The humor hinges on a bait-and-switch. “We were incompatible in a lot of ways” sets up a conventional, almost therapeutic explanation for a relationship ending; “I was a night person” sounds like the start of a mundane example. The punchline—“and he didn’t like me”—reveals that the incompatibility is not a neutral mismatch but an emotional rejection. The joke satirizes how people dress up painful truths in polite language, and it also turns the speaker into the butt of the joke through self-deprecation. Its sting comes from compressing a complex breakup into one stark, unflattering fact.