Because they can. You wouldn't let anyone else hurt you that bad...
Love moves in a circular fashion. When you have reached hate on the wheel, you are the closest to being back at love. It is like seeing your own reflection at the end of the universe. Creepy eh?
Because to love someone is to put honesty, openness, emotion, etc, on the table..if you lose, hell yell, it's going to hurt. The ones closest know your weaknesses..be aware of those that you allow to be that close and you will be just fine.
you are more vulnerable to the ones you love.
Remember the old saying: You always hurt the one you love.