As a child, I unfortunately had family members with a poor medical and physical history. I was quick to recognize that having a body which could not withstand a fall, a surgery etc.. was something I could help avoid in my future and hopefully in my own kids futures someday. I decided to compete with the bigger, older and stronger kids which eventually led me to want to workout. I waited a long time to get into the gym, but loved it ever since.
Too many reasons to list. #1 is physical health. I feel great when I am not working out which in itself is woth it. Of course, throwing around the weights brings me that rush which you just can't fully put in words for someone who has not experianced it. Odds are, if you are on this site, you know what I am talking about. Also, you build confidence not only in the gym, but inn all aspects of life.