I mainly go to the gym to stay healthy and in shape. I have an irrational fear of developing cancer or a neurodegenerative disorder (MS, parkinsons, alzheimers, etc.). Thus, I try to stave off any of these issues as best as I can with fitness and eating right.
As a natural, I do not expect to be a monster. As long as I can keep making some gains, no matter how slow and steady and still look good when I am 50, then I will continue to go to the gym. I can't be young and totally fit forever, however I do not want to be the 70 year old man who can barely walk up the stairs and needs to have someone wipe my butt for me. That's enough motivation for me.