Loving someone who can't be yours is probably one of the most painful thing to experience. You keep yearning to be with that person, with the knowledge that you can make that person really happy, if only he will let you, that you are the perfect one, if only he will realize it. You love someone who is not yours, and most likely someone else is like having that really really favorite food in front of you but at the same time knowing that if you take a bite, you'd get poisoned. It may be worth the bite but you know that you can never do it.
When you love someone who will never be yours, it will always be bittersweet. There may even be a point where you just want to not love him because it hurts too much, because you might want too much, more than what the other person can give. When you love someone who will never be yours, it is the ultimate sacrifice; you know he will never love you yet you can't stop loving him.
When you love someone who can't be yours, you stop loving yourself.