Why is it that there's no beginning of a dream?

is once stated by Leonard Dicaprio in Inception.
Now that I think about it,
there really is no beginning in a dream,
at least, not that I know of.
I’m always in the middle of my dreams.
BTW, these days I’m too tired to remember dreams,
not physically but mentally so much so that I don’t have any
energy to get involved in things happening in my dreams. : -)
So, I’ven’t remembered a dream for like 3 years in a row,
which is fine by me as most of my dreams are composed of nightmares.