Letting go of someone

I dont understand the thought in my head. Im not jealous or anything. But everytime I see her with another guy, I feel bad, angry, sad... It's not that I have a reason to be jealous, she's not my girlfriend. She's my friend. I cared for her as an older brother would care for her younger sister. Protecting her from harm, keeping her happy and smiling at all times. But when she met him, I guess thats how a father would feel when he sees her little girl grow up and let her roam throughout the world that she lives in. I guess this is how I must feel... Letting my little girl grow up, eventhough she almost my age... Letting my friend move on to her world...