how can the one's we love, hurt us the most?

why is that even a question?
if someone loves you with all their heart, surely they will do anything possible not to hurt you right?
or is this even possible?
as two humans brought together can there be love without pain and heart ache?
do animals love each other for life, is there a fate or destiny that keeps one magically together with another?
or are we like animals and are meant to move from one to another to experience different joys and lessons from each other?
what is LOVE?
why would love HURT?