I can sum up in three "acts" the breakdowns and breakups of most relationships since the beginning of time:
Act 1: You hurt me.
Act 2: Because you hurt me, I now hurt you.
Act 3: Because you hurt me, I now hurt you and so you hurt me again and so I hurt you—and downward spiraling we shall go.
John Gottman, the famed founder of The Love Lab (a family research laboratory where where couples are studied), says he can consistently predict how long a relationship will last, not based on how well a couple gets along, but by how well a couple doesn't get along. A relationship is only as strong as how well the two can deal with their weakest moments and how well they handle conflict.
3 ways couples deal with conflict