Does anybody know why dreams have the power to make you feel or believe that an event has occurred, when in reality it hasn't and then once you've woken up you're either confused or angry that it really didn't happen.

for example you could be dreaming that someone you kow has died and you totally think it's real but when you wake up you're still trying to figure out what happened. Or when you're dreaming that you know something important, but then, to your dismay, you realize your mind just completely fooled you! That's been happening to me alot lately and I was just wondering why? Is like the logic part of you brain turned off, or is your brain just easier to influence in REM?