You start to fill up your days with things that you want to do. When you are working, most of your time is devoted to the job and there is a tendency to put off things that you really enjoy. Once freed up from the job, you realize that there are so many things that you want to do, and there isn't any excuse not to do them.
I don't think retirement is for everyone. Some people don't have interests outside of their job. Their job gives purpose to their lives. They have a tough time adjusting to retirement. Also, people who love and live their jobs shouldn't retire.