Because they know that no matter what they do, you'll forgive them.
That's true. For the most part, anyway. They'll wrong you, and hurt your feelings, and probably offend you more than once. Time after time you'll accept their halfhearted apology and go on about your business, until one day they just push you too far.
Once you hit that point, there's no going back. There's no more forgiveness left.
And once you hit that point, it's time to realize that they weren't worth your time to begin with. Of course you'll be disappointed at first, and of course you'll miss that person at times, but a person can only give so much when all the other person does is take.