Why do you forget your dreams after you wake up?

Because they are put into your short term memory. If you want to remember your dreams you have to try to recall them the first 5 minutes of waking up, or they are usually completely forgotten. You have about 10 dreams a night so it's difficult, you can only remember every one if you woke up during each one. And also you usually can only remember the most recent ones you had prior to waking. Hope that works for you:) haha