Guy best friend ruthlessly insults his girl best friend's boyfriend's appearance: 'His hair is greasy, the tan fried his skin, and he's just really hideous and unpleasant to look at.'

Is he just plain evil, or is he jealous because he likes her? 

If someone is constantly spewing judgments about other people's appearances, it reflects really badly on them. When you're a kid, it's difficult to understand that the kids who make fun of other kids for their looks are just insecure about themselves, but it really is true. Your mom wasn't lying when she said that other people's insults about your appearance reflect badly on them, not on you. You learn it once you become an adult and see how much haters actually hate themselves. 

We've all had friends who have dated people whom we wouldn't necessarily date ourselves, and thank God for that! My life would be pretty complicated if I thought all of my friends' boyfriends and girlfriends were the most attractive people I'd ever seen. Even if you think your friend could date "do better" than the person that they're dating, you'd be smart never to say that out loud to them ever for as long as you live. Chances are, your friend sees the beauty in them that you don't see, and as long as they're attracted to them, your opinion on their appearance really doesn't matter in the slightest.

It's not uncommon for jealous and possessive people to dislike their friends' romantic partners because they secretly wish they were in a romantic relationship with their friend.