Relationships can be so complicated; high highs of love and laughter, a sense of belonging, and low lows of stressful arguments, trust issues, and frustration beyond belief. When things get bad and there's an argument, there's two directions it could go in. The couple could look at it as an external issue that they tackle as a team, or they could decide someone is right and someone is wrong in the situation. Now, it's not a very healthy tactic, but boy is it common.
The ways in which we argue can have immense effects on if we're going to last as a couple or not. For couples that have too many fights, issues, or betrayal, then it usually means it's a doomed path. For couples that take on issues together, well they're more likely to make it. In the event that a couple does break up, there can be a myriad of reasons why. There can be obvious reasons, like betrayal or long distance, or there can be underlying reasons that simply amount to things just 'not working out.' But when someone feels slighted too much, then it could turn into a passion for revenge.