Evolutionary psychology is behind one of the primary theories, in the simple way that it’s prudent for us to be wary of things that might hurt us. Bugs can sting, bite, invade, cause sickness/death, or poison us.

There’s also the disgust aversion angle, which is tied to the relationship between a lot of bugs and indirect environmental threats (also ties into evolutionary reasoning) like rotting things, or corpses specifically.

In the broadest strokes, we associate bugs with pain, disease, death, and decay.