Keep in mind that he is coming off a race that he just won, so he was probably on top of the world and loving running. Very few people want to think about their careers being done after such a strong performance.
Assuming that he is truthful about his desire to return, this move is very reminiscent of Michael Phelps. After Beijing, it was obvious that Phelps was one of the most dominant—probably the most dominant—swimmer in history. He won eight gold medals in the pool, and many people wondered what he still had to prove.
However, he came back, presumably to shatter the record for most total medals won in an Olympic career, and he did that. Now, there is absolutely no doubt that he is the greatest swimmer of all time.
Similarly, you might wonder what Bolt has left to prove. All of his world records are definitely a testament to the fact that he has dominated this era.
That being said, over his entire career, he has still only won five medals. I don't mean that in a dismissive sense, but I'm just trying to point out that Carl Lewis had 10 Olympic medals.
What athlete doesn't want to try to break every record in his or her sport?
Bolt would need at least two more Olympics to break that record unless he picks up a fourth event.
However, sprinting is one of those sports where age catches up to you rather quickly. It will be interesting to see if the best sprinter of all time can even stay ahead of this sport longer than almost any other has been able to. He easily has the best chance in the world right now.