Why am I so possessive over my friend?

Hi everyone. I have a weird problem that is really taking a toll on me and I know it's wrong but I really can't help it.

I'm extremely possessive of my friend. I have no clue why. He's like the big brother I never had and the thought of him becoming close/closer to someone else, especially another girl, makes me want to both break down in tears and fight (the other person).

I don't have feelings for him, at least I don't think I do. I have a boyfriend whom I love very much. But something about him just makes me want to keep his friendship all to myself. My best friend, whom I tried to keep from him because of fear that they'd be close, has met him and I've literally began almost hating her because I feel like she may take him from me even though that's a ridiculous thought because he's not some object. This has pushed me from seeing a girl I've called my best friend for a year as an enemy. This is how serious this is.

He's beginning to catch on and of course he's not keen on it. I hate being like this, he's a human being and deserves to be close to other people, even girls, besides me but I just don't like the idea. Help me see the light and know why I'm like this.