Bestfriend and boyfriend? 10 points for best?

So my bestfriend which is a guy got tired of me talking about my boyfriend being so over protective and crap so he wrote him an email telling him to eff off and grow a pair and that if he'd ever try to hurt me he'd crush his skull. He used to like me 2 years ago for 3 years and now he moved and we started getting close again and he does this.? What does that mean? I mean he said he out grew it all but why us he defending me like this? He refers to me as his sister but I was talking to him once when we started talking again and he asked why I don't dump my boyfriend for him.