The reason for this is that, should that person not return your affection, you do not fall out of love. If you lose the feelings you had, it was never love to begin with. No, once you are told that there will never be any chance of having a future with the person who has your heart in thrall, a horrid truth will rear its terrifying head.
You are devastatingly more in love with that person than you ever were before. Every amazing quality they ever displayed is now coupled with the attraction that comes from unattainability. An entirely new dimension of magnetism is added when the object of desire will never be the object you can call your own.
Tragic, isn't it?