The Persuaders taught us "it's a thin line between love and hate" and they were so, so right, because no matter how much we love a person we can turn around and be a real jerk to them at the drop of a hat.
Perhaps it's passion that drives us to do our lovers wrong, or maybe it's because the wounds inflicted by our partners are deep and never seem to heal, since we let our guard down and invited them into our hearts.
Wife moved out on my birthday and in with her boyfriend...3 years later I was a collector for a bank and found out that they both were hiding their cars so they would not be repossessed. I worked with their lender to have both of their cars repo'ed at the same time in two different cities. Yes, I hold grudges for a while....
For whatever reason lovers always have and always will pull the worst dick moves on each other, moves which feel right in the heat of the moment but may make us deeply regret our actions later. Then again, if they cheated on you without remorse they probably had it coming!