Why is it that good dreams, are just that, very dream like? But nightmares, and night terrors.. are more real than real life?

I don't know where I am going with this. All I know is that for the last several nights, I have had some very excruciatingly horrid dreams. So real and life like that I have woken up only to tears streaming down my face, and feeling exactly as I did while I was still asleep, but not quite sure if I was really asleep and questioning if it didn't really happen. Those are so disturbing. So scary, so real. How come I can't dream a wonderful dream, and wake up feeling that good?

Okay, well I will go for now. Just wanted to put this down.