"Also a little regret knowing they will never go back."

Wow, I had never thought of that aspect of the Apollo program. It's a little sad that of the 22 people to orbit the moon, land on the moon, or both, it appears just four of them ever made a subsequent space flight.

From Buzz Aldrin's bio on wiki:

"In March 1972, Aldrin retired from active duty after 21 years of service, and returned to the Air Force in a managerial role, but his career was blighted by personal problems. His autobiographies Return To Earth, published in 1973, and Magnificent Desolation, published in June 2009, both provide accounts of his struggles with clinical depression and alcoholism in the years following his NASA career."

I think they had an informal policy of letting as many different people as possible experience space.

So you trained for years to go just once. Not the best use of money, but I can easily understand why.

or maybe they were just fresh out the steel capsule they had to live 40 days