Why is it bad to be disrespectful during mass?

The point of mass is to honor God.

So when you are disrespectful, those that are there for that purpose (or even those that are there just to save face, but are fully aware of the social norms of the situation) will become angered/annoyed.

If you don't want to go, don't.

If for some reason you have to, do so quietly. d