Adults participate because they were brought up that way, or discovered the lifestyle on their own, through a friend, through a family member. Allowing yourself to accept your body in its natural state is a return to innocense. It is child-like in nature. It is also a state of mind that carries into all areas of your life, personal relationships, and societal views. For others, it's just being nude because it feels good not to have clothes on.
Different nudists have different reasons for being nudists.
I also believe that everyone is a nudist at heart. They just may not know it yet. There are plenty of web resources and numerous organizations. You can find them if you explore this website. Registration is free and all information is kept confidential.