Nightmares in Random_Thoughts

Why do we sometimes wake up and not remember our dreams and on other nights we startle awake and remember every aspect of our crazy scary dream we just had and we mix the reality around with the dream and we are scared that there is something lurking behind the corner.
It is so scary to burst out of sleep totally sweaty and thinking you are still in the dream you just had. Wish we could change that and make it better to sleep. Just to forget all about everything.