We get bombarded with endless information, so the brain has to be selective, tossing out a memory unless it's told, "This one's important, keep it!" To be clear, we're not talking about disease or injury or age — just a normally functioning organ that prefers tidiness. "Without forgetting, we would have no memory at all" is the way assistant professor in psychology Oliver Hardt of McGill University puts it. It comes down to clutter. If a brain stored everything, it would be jam-packed with useful info, plus mounds of mental tchotchkes.