I cant in good conscious just show the answer, cause its my firm believe everyone read this book series without it being ruined. So I've spoiled my post, though you can read the sum up if you dont want to have it ruined or dont want to know my real reasoning.
<spoiler=post spoiled so no one is pissed>Well, if you read, actually this time, he gets the Horn of eld, which he lost in the battle that claimed his friends. So history has changed, even minorly, meaning that this time, things will slightly be different, and he is on that path and one step closer to happiness. Or at least fixing things. Now, as to why he starts over, its because as you said, Ka is a wheel, and a wheel is endless. It doesnt stop like a square and then make a radical change. it makes slowly curving changes, ones that are subtle and unimportant.
I also think its just kinda SK's way of giving you that big middle finger and saying you decide what happens damnit! Like he seems to like to do in most of his books, since there's never a clear and definite ending. Its something like how evil will always exist, and so will the good that combats it, so whatever you think will happen is the best way to read the book. I mean, just loook at his other works.
<spoiler=Endings to his other books>The Stand - Flagg survives
'Salem's Lot - Vampyres are still around
Desperation - the evil is only buried, not killed
Sun Dog - The dog comes back, through an eerie message that I still like to quote and is one of the scariest things i've ever read.
If you didnt read the Spoiler and want the meet of it, I just dont think SK likes to end his books definitively.
EDIT:
Jamieson 90 said:
The point is this someday Roland might actually realise that the people around him are more important, and he wont care about getting to tower anymore, and therefore he wont need to be sent back.
Not to say you're wrong, but thats not likely to happen. Roland is honour bound to go. He promised his father, his friends, and is the last true knight of the world. He is in a sole pact with god and all of existence. If he doesnt go back, its all over, and Roland, though not completely ruthless that he doesnt feel remorseful for his friends' passing, will not stop.
King summed up Roland most powerfully when he said his eyes were faded blue from the sun and travels. That alone tells you the conviction this man has to go on. Now, he may fail, but he will never stop. Cause I'm pretty sure:
<spoiler=plot event>Losing two fingers
is rather powerful enough deterent to do anything.
