Why does love hurt so much?

I have been in love with the same girl in my year at school since I met her Three years ago, I liked her from a primitive stage which has progressed over the last Three years. But this year I have really fallen in love with her, I can never stop thinking about her, I always look to see where she is at school, she is one of the only things I always think about, and I always miss her when I am not with her. She considers me just a friend, and I know if I do get to be in a relationship with her somehow (miraculously), it will never work because of various things. Anyway lately she has become really close with this guy in my year, every time I see her, she is always with him, they go everywhere together, even out of school they are always with each other, it hurts so much when I have to see this, and she knows I like her because I recently told her. She is so beautiful, and I have tried to get over her but it is just too hard, I realise I am in love with her, but what do I do? they are always together? its so upsetting. Love hurts so much, especially when it is not mutual.