Why is my brother so overprotective of me?

I'm 15, he's 17. It's seriously so embarresing. If a guy even flirts with me and he sees it, he'll pull hil aside later and say things like "stay away from her" My brother has his friends come over and stuff all the time. (note, some of them are players). The other day I was sitting in my living room, he had like 6 friends over and I overheard them talking and they were talking about "bro, your baby sister's really cute she's so innocent blahbah" and my brother's like "DON'T EVEN THINK ABOUT IT" They were JUST complimenting me! The worst part is my brother's a player too! He sleeps with a bunch of girls, what's the big deal if a guy FLIRTS with me! It's like guys don't even come near me anymore because of him! How do I get him to BACK off? Why is he doing this!