text: Memories don't last forever, some disappear on their own.  It's a commonly held myth that every sensory experience you've ever had, every thought scent dream touch etc . . . is stored somewhere in your brain - research on memory suggests the opposite.  We filter most of the incoming information out and erase old experiences to make way for new info.  Memory is not infallible either - crime victims often get important and obvious details wrong, and memories can be "created" through suggestion (look up "repressed memories" and "exonerated child molesters"!)\n\nThe memories that people want erased, however, are typically the ones that don't go away.  A characteristic of a traumatic experience is that it gets so firmly written into a person's brain that it just doesn't go away, and the experience keeps coming back, over and over again, because the brain has somehow determined that this is an important memory that can't be ignored or written over, and that's not always a good thing (Post Traumatic Stress Disorder).  Though memories like that can be lessened over time and with the proper therapies.