Why do we forget some dreams after waking up?

We forget many dreams because the brain often makes them while memory-storage chemistry is turned low. When you wake, your attention snaps back to sounds, light, and plans, so a dream that was never strongly filed can fade before you can name it.

Dreams with a big feeling, such as fear or joy, are easier to remember because emotion gives the brain a stronger reason to keep them. A child who lies quietly for a moment and tells the dream right away may catch more pieces.