Anger vs. Gratitude

I've observed that, as human beings, we have a tendency to remember situations where we've been angry much more than when we've felt gratitude.  For example, we probably will recall -over and over- a driver who didn't give us room for a right turn or one that kept us from merging on the freeway. 
However, when we look at situations of gratitude, they only seem memorable for a few minutes. We may have felt grateful for the person who let us go first at the stop sign, when the right-of-way wasn't ours, but we soon forget this incident.
Some people might have trained themselves to focus on the positive incidents in their lives and to "forgive and forget" the bad ones.  The rest of us should inspire to be like them.