why are boys shitty?

So I'm not one that talks to or dates a lot of guys. I've only dated 3 in my 19 years of life. But yet for some reason EVERY guy I have been involved with has started trying to talk to my friend after he ends things with me! Like what the fuck!?! It's been three different guys have al gone after the same friend? Have any of you had an ex hook up with your friend? Why do they do this! Do you think they're purposely trying to get back at you?