Why do the negative aspects of an experience stay in your mind instead of the positive aspects of the same experience?

How the mind works, how memories form, how they last, why they are forgotten is a branch of study that is far from complete. We know some things about memories but not everything. There is a theory that you remember negative things more clearly and for longer because it is a survival trait. It is more important to remember that fire burns than it is to remember that flowers smell good. Another theory is that the chemicals in the brain build stronger pathways when your emotions are negative. These two ideas don't have to be seen as conflicting.

What is known for certain is that the more often you think about something the less likely you are to forget it. If you want to remember good things, think about your good memories more often! (This also works for French class or Algebra.) Hope this helps!