One of the worst feelings in the world, when you love someone and they don't love you back. This could be because they only see you as a friend, or they just don't see you at all. You could like them so much and think about them all the time but they will still carry on going out with some whore and isn't good enough for him. It really sucks. Maybe one day you'll get over it, but for now, you feel like your heart's being shattered into a million pieces and you are incapable of making an effort in moving.
Billy will never love Susie. It's an unrequited love.