Some people are simply negative. And that's it. Never a nice thing to say. They can't open their mouth with a criticism or a nasty thing to say. I call that Toxic.
It's even worse when it's an adult and it's aimed at children. Seriously. Does it make the person feel really good to pick on a child all the time? It must be, because I can't see any other reason to do it.
To step back from a situation and see that 99.9% of the time the response /reaction is a negative one you get to thinking "There's something wrong there."