In your opinion; why do people work out at the gym vs. being active outside? ?

I'm composing an ethnography about the subculture of gyms and exercise facilities. I would like to get some information about why people go to the gym as opposed to being active out of doors or in your own home. How often do you use your gym membership? Do you enjoy going to the gym? Does it make you happy with your outward appearance? If your a personal trainer, please elaborate on why you chose that specific profession, do you enjoy it, etc..?