1. When people complain about being fat or ugly, they often are looking for back pats and for someone to tell them they're pretty, so if you say either of those things chances are someone is going to tell you you're fine the way you are.

This, homigod. Or commiseration. In my case, I usually I default to vague encouragement, like "you'll succeed at whatever you want to do about it", no matter what they actually say about their weight. One thing people rarely want honest opinions about is weight, and I have no desire to provide what isn't wanted.