Why would a guy brag about having sex with women to another woman?

So there's a guy in college who was recently dumped by his girlfriend, and so he's clearly on the prowl. It's obvious to me that he finds me attractive and wants to hook up. The last time I spoke to him whipped out his phone and showed me pics of a girl who he hooked up with, and was like, oh yeah, she was all over me. He then started talking about some other girl he hooked up with Freshman year, and who his friend is hooking up with.
Why would a man say that to a woman? It's not like I'm his bro whose going to pat him on the back for fucking someone. It's weird, and I'm not impressed.
Why would a guy brag about having sex with women to another woman?
Post Opinion