Of Mice and Men ends where it begins, with George and Lenny alone by the pool. Why does Steinbeck choose to end it that way?
You would have to ask Steinbeck directly for sure why it ends that way. As for myself, I am willing to speculate a few reasons.
First of all, George chose the place for it's location, and it turned out to be quite peaceful. If you take a look at the cycle of life, quite often our entrance to this world and on our way out is just that peaceful. A newborn baby sleeping is one of the most calming things to look at, and a person relieved of all pain and suffering in this world must be equally at peace.
Secondly, this place happens to be a memory marker that George intentionally made. Every story has an inciting incident that sets the story in motion, there is eventually after much complication a climax and then a resolution. The inciting incident here may be Lennie's problem with liking soft things. By the end, we see where that takes him. The resolution is often a direct answer to the inciting incident. This location offers the ultimate in solace and comfort as George speaks softly of the place they are eventually gonna get.
Steinbeck's choice is to make a cyclical story whole, and address the issue of ultimate peace, a very necessary theme in Great Depression era America.