Why is human memory less reliable than we expect? Perhaps because our mental model for how human memory works is wrong. We picture ourselves retrieving a memory from a data bank like a computer retrieving a file.
But MRI studies conducted by Daniel Schacter indicate that the way we remember something may be the same process we use to imagine the future. That is, our brain takes disjointed components and tries to assemble it on the fly into a coherent picture. A process like that is inherently susceptible to influence. We may imagine a rosy future for ourselves for a variety of reasons, but it may be just as likely that we wear rose-tinted glasses when it comes to our past as well.
Our brains process time in one direction, but if we were to reverse the direction, perhaps it would feel the same to us. That is, we'd be remembering the future and imagining the past.