Why does my brother not want me to date his friend?

girlie_girl
One of my brother's friends and I have been flirting a lot lately. I like him, and I think he likes me too. I'm 16 and he is 19. The only problem is that he is one of my brother's friends. The other day I asked my brother what he would think if his friend and I were to get together... and he pretty much freaked out. He said that he would never ever let that happen.

Why does he have such a problem with that?
Why does my brother not want me to date his friend?
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