Why can't we remember all our dreams?

Someone has told me that everyone dreams everyday of their life. Is it true? I know that sometimes when I wake up in a middle of a good dream, I always try to fall back asleep to go back to the same exact dream. Most of the time I can go back to the same dream I was dreaming. Sometimes I even know I'm dreaming in my dream. In the dream, I told my friends that they were in my dream. Another weird thing I've had was I would wake up and think I'm awake but then I find out that I was still dreaming. I remember some of my dreams but as the day goes by, when I try thinking about what I had a dream about, I can't remember. So the question for today is "Why can't we remember all our dreams?"