If you read his book, you'll learn that he had nothing to prove to anyone but he knew he still had one more slam left in him. His last 2 years on the tour was all about winning that last slam he knew he had, so when he won it, he had nothing left to prove to the world and more importantly to himself. The next morning after he won the USO he was on a high, he was basically on a high until the AO, then he had no motivation to play that and pulled out. He was totally content.
At that stage he had pretty much done it all, except win the FO, but he had resigned himself to not winning that 5 years earlier, but was no big deal to him because everyone since Laver had failed to win a career slam. But, he had the most slams, the most Wimbledons, a record 6 years ending number 1 and he had the best of his main rivals. What else was there for him to achieve?
Plus, he was in a serious relationship at that time too.
That is all.