MEMORIES TAKE UP RESOURCES, AND MOST MEMORIES LACK SURVIVAL VALUE: To Remember, the Brain Must Actively Forget: Researchers find evidence that neural systems actively remove memories, which suggests that forgetting may be the default mode of the brain. Which raises the question of why I can remember cigarette jingles from when I was 5 years old.

Actually, I have a theory on that. For the vast majority of evolutionary history, people were preliterate, and songs were the chief means of longterm information transmission. (Rhyme and meter, etc. even serve as primitive error-checking mechanisms.) So being able to remember the songs that told you where to find famine foods, etc., had definite survival value.