We tend to go into relationships for all the wrong reasons, and as a result we're too demanding of the other, too possessive of them, we expect them to make us happy and I don't think that's their rightful job. Maybe we got into relationships for the right reasons then breakups would be friendly and non-hurtful? I don't know. 'Just me' makes a good point, they become a part of you and you become a part of them, so losing them is like losing a part of yourself. And everyone wants to love and be loved, when a lover leaves that closes a major door on the opportunity to do that. I can see how it might cause grief even in what I'd call a 'pure relationship'. But would it necessarily? I don't know. And would it cause as much grief or pain as an unhealthy relationship following the typical social patterns? I think not.