When you love someone and little by little they stop wanting to see you because, secretly, they want someone else...When you find out, that just crushes you.
And also when you are with the one you love and they don't just see you...they see everyone. And I don't mean just sleeping around..
When they know you are hurting and have low self-esteem, but continue to do the same things over and over.
When they know just how to break your heart.