It's about ownership for some guys. When they have a girlfriend, they think she is their property. When the girl does something that contradicts this belief, the guy loses his mind. I've seen it happen a number of times in my own life -- not to me, but to others I know. My older sister had a boyfriend. Nice guy, but when my sister told him she didn't want to see him any more, he went nuts. Started to stalk her. I think he even contemplated killing me to get her attention. He didn't do anything, but I got that vibe off him once when he picked me up in his car and started asking questions about my sister. It was creepy, to say the least.