He assumes every decent looking guy who speaks to her is trying to steal her. He sees her as his property and has poor views as women, but he also sees you as a threat and untrustworthy because he thinks you'll go after "his girl".
It comes from insecurity and a lack of self-worth that he's taking out on others that aren't himself.
My best mate in work, he was friends with one of the staff for years. I knew her well through him but not that close. They'd hang out a lot at work, and always set up group things with us all. It was never romantic, but then she got a boyfriend who felt threatened by him, and that was the end of that.
