People have insecurities and egos. Sometimes you come across someone who need to feel useful, or need to feel important, or have a need to assert their power. Or that simply don't trust you, and assumes that if they don't see a problem, something is lurking under the surface.

With a lot of people you will never need this. But especially in larger projects where any number of people might latch on and review submissions, the chances for someone to find something to complain about goes up rapidly.

And when one person has found something to complain about, all kinds of other social dysfunction quickly becomes an issue too.