The memories of dreams evaporate remarkably quickly. Much faster than anything else. It can't be accidental; I think our waking brains must be purging those memories as a matter of urgency. If we remembered them, we'd "learn" from them, which would be a catastrophe.

You know how you try to run in a dream, and you don't get anywhere? Imagine if your motor cortex started to expect that, when you're awake. It would destroy you.