We can see that there are many ways in which we actively contribute to our own experience of mental unrest and suffering. Although, in general, mental and emotional afflictions themselves can come naturally, often it is our own reinforcement of those negative emotions that makes them so much worse. For instance when we have anger or hatred towards a person, there is less likelihood of its developing to a very intense degree if we leave it unattended.However, if we think about the projected injustices done to us, the ways in which we have been unfairly treated, and we keep on thinking about them over and over, then that feeds the hatred. It makes the hatred very powerful and intense. Of course, the same can apply to when we have an attachment towards a particular person; we can feed that by thinking about how beautiful he or she is, and as we keep thinking about the projected qualities that we see in the person, the attachment becomes more and more intense. But this shows how through constant familiarity and thinking, we ourselves can make our emotions more intense and powerful.

— Dalai Lama

from The Art of Happiness