When someone treats you unfairly or unkindly, it is only natural for your mind to be flooded with resentment; everybody's mind works that way. But the day can come when you scarcely remember what resentment means. It is not that you will be blind to others' behavior, but that negative responses will not even arise in your mind: and when negative thoughts do not arise, you respond to every situation with love. This is what it means to be "pure in heart."

Eknath Easwaran, Original Goodness