An idea came to me while I was waking up.
I think there are two possible points of view on vanishing dreams. First is that bad dream recall is a side effect of some fundamental process in brain. And the second is that bad dream memory is nesessary by itself and it could be naturaly selected.
And I think that bad dream recall protects us from recalling bad dreams. The nature is blind and it doesn't distingwish between good and bad dreams. But the individual does. And the ability to recall bad dreams clearly may influence bad when waking up. So it is better to remember nothing.
For me this is the simplest explanation.
But i don't know how to verify it.

What do you think?