Why don't you remember dreams

the best way to remember dreams it to keep a diary.
as soon as you open your eyes use the book.
dont do anything else.
after a time you can remember they well.

I think the human brain stores memory in a active form.
the memory moves about all the time.
and when you dream your minds sees them pass.
but it is crazy.
so the mined makes us stories from this.

it could be the mind is just defragmenting.
like a computer does?