I know exactly why; I just can't remember.
If you had posted this yesterday I may have been able to help, but now I cant remember.
Apparently you only remember dreams if you wake up while still dreaming. Or so I have heard.
I remember almost all of my dreams and if its a weekend or a day i wake up and fall asleep many times until i finally get up.. i have a few dreams and usually can remember them all.
I used to never be able to remember my dreams but one of my profs at uni told me this: Keep a writing tablet and pen by your bed and write down the first little fragments of thought that pop into your head when you first wake up. Eventually this will lead to your remembering pieces of dreams and you will be able to train yourself to remember whole dreams. Try it!
I hate when you wake up in the middle of an exceptionally good dream that seems real. I used to have a lot of dreams of me and this guy i used to like. It would seem so real and when I'd wake up I'd have to really think to remember if it was real or not and it always ends up just being a dream :-(