I think it depends on -
1) Who was the person who invested the most in the relationship (emotionally, physically, financially, etc.)?
2) Who saw it coming first?
It is not always the person who breaks-up who saw it coming. Sometimes, the person who initiated the breakup may have been forced to do so by the other person's behaviour. Let's say, A and B were in a love-hate relationship (kinda like the one Rihanna and Eminem croon about, in "Love the way you lie, Part 1"), where A was the abusive partner. After putting up with A for a loooong time, B finally decided to call it off. Neither does it mean that B was the first person who saw the breakup coming, nor does it mean that B would be the least affected between the two. It is possible that B may have broken up with A, and then spent a week or two crying over it.
Also, in the same context, merely because B was the one who brokeup with A, does not mean that B was the person who invested the least in the relationship. You see, that is why I mentioned my first point, because, in some relationships, the person who invested the most may also be the person who chose to walk away from it, but that does not mean they saw it coming.
All said, relationships are tricky and there is no failproof method to protect yourself from being hurt or from even hurting someone. We have got to understand and accept that, as humans, we are inherently messed up. No matter what our best intentions may be, there will be situations where we end up hurting the ones we love. All that we can do is to minimize the risk of such situations happening by looking out for red flags (inconsistent behaviour, flimsy standards, etc.) in the ones whom we fall for.