For me, it would have to be if they back-stabbed me in any way possible. Revealing secrets or gossiping/talking behind my back or really just trying to push me away. Now, sometimes people need space and I admit, I can take that to the next level but I personally don't like it when someone pushes me away, I have a natural instinct to just help people, even if they don't want me around. But I mean like, it's not like my dog is ever gonna do any of that stuff so why am I worried?
I'm kidding, I have real friends but I would have to say my dog is my closest friend, I've had her since I was 2 so we basically grew up together.