Why does unrequited love hurt so bad?

I fell in love with a guy a couple years back. He was never my official boyfriend. We'd see each other all the time & actually became really close. He knows me very well & vice versa. It was a passionate & intense relationship he just couldn't commit to me no matter how hard I tried to change his mind. I stuck around cried, begged and sworn to never speak to him a thousand times but I'd still go back, or let him back in. He knows I truly love him. And maybe he loved me back at some point but I'm tired of feeling unwanted so I'm staying away. The thought of him triggers my deepest emotions. Why does it hurt so bad?
Why does unrequited love hurt so bad?
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