You don’t have to believe their words. But arguing against their claim of suffering is where the line is crossed of being a dick about it or being a friend there to listen to them.

Either way your eyes don’t see what is inside someone’s mind, so why are you placing so much trust in their judgement? Why does even it matter, and why do you feel the need to deny something you can’t prove anyway? Just want to argue?

To answer your original question, it’s because it makes you insufferable.