Negative emotions linger longer. Why?

 Why is it that negative emotions and feelings are so hard to get rid of?  They pop into our minds intrusively and regularly without any sort of encouragement and make us feel bad, often when we are trying to relax, interrupting sleep, appetite and other pleasurable activities.

On the other hand, pleasant, happy and positive emotions and feelings are much more transient, disappearing quickly and easily, even though we would like to hold on to them.