There are people in my job who do a full thirty years then come back as support staff. It's usually because they either need to keep working to pay for mortgage/kids etc or they have got used to a lifestyle and don't want to change it. Good luck to them, I say.

For myself, I view retirement as a chance to change life, to do what I want to, not what I have to. I view three stages of adult life; Twenties is a time we have the opportunity to get out and sample life, have fun and take risks. Then with marriage/kids/mortgage comes responsibility so most settle down to a job to pay for them. Then kids grow up, a chunk of mortgage is paid off and retirement means freedom to go out and enjoy life again!

A job is just something you do to pay for real life. You can't buy time, no matter how rich you are.