Why don't dreams have an end?
I am not talking about just any random dreams, but ones where it is kind of telling a story. Some you just walk around doing stuff but some have a real obvious story to it. I have had story dreams where they end but like 98% of the time they don't. Even worse, most of the time they end just when its starting to get good.
Also if your a person who normally gets up at the same time everyday, you will know that your body can tell what time it is. If you get up at 10 everyday, theres a pretty good chance you will wake up then without even trying. So it sounds like your body knows how long you will be sleeping for, so why would it plan a story dream that it knows you will never finish?