Is it possible that this whole story is a sort of purgatory for Roland? Maybe he has already died, and each time he goes through this quest it cleanses his sins and teaches him to be a better man. He clearly becomes much more emotional and attached to his ka-mates throughout the series. Maybe when he finally completes his journey and has cleansed his sins, he'll find the "Clearing at the end of the path" when he walks through the door at the top of the Dark Tower. Maybe his real life was similar to this journey, but he was forced to complete it again and again until he was ready to enter the clearing. Or maybe his real life was something completely different, and all of this was just his own afterlife.