Why is it offensive to call someone a dog?

cause some people dont like it

According to the Urban Dictionary, "dawg" is a term of endearment meaning "close friend" or "friend."

Where I come from it is only offensive to call a female a dog because it is an old slang terms for ugly or a **** (as in "mans best friend").  To call a guy a dog (aka "dawg") refers to him being a "hound dog" who sniff's out girls/women and suggest he does well with them.  It can also refer to a guy who does well (or works hard) at pretty much anything.

Depends if they mean it in a bad way or not. Or if the person you are talking trusts and likes you.. Matters to. I use to have a nickname called.. The dog.. I'm guessing because at the time i use to never leave the house. I remember this one guy saying he was gling to take his.. And he used my name.. Trying to talk about taking his dog for a walk.. Lol... Oh well... I dont really mind unless I'm trapt having to listen to people trying to bully me. Otherwise I'm pretty peaceful and not problematic.

It's not always, dog.