Why is it so hard to let go of someone you love?

im ashamed to say its been two years since we broke up and shes still on my mind. though it got progressively less painful as the years went by. we're friendly towards each other now after a really horrible fallout that lasted a year. and im happy that shes met someone who makes her happy. thats all that matters really. but its still hard, you know? loving someone without expecting anything in return