When we give our heart away, why do we get surprised when it hurts?   
The term "give our heart away" means that unless we have someone giving us their heart in return, we're left empty and hollow and that's painful,, and when we're such lovely and loving people, we need to have that exchange or we slowly but surely diminish ourselves...
It's excruciating work to recognize that we betray ourselves when we demand someone else make us happy..