A family member got me into working out when I was very young. He is an ex marine and gave me a small weight set and bench followed by a full body workout routine on a poster that he made for me. Then it was weight training for sports that got me into working out consistently and regimend. I always acceled in hypertrophy compared to others in the weight lifting programs. I was no where near the strongest or the biggest, however. Most people considered me small, although I was always "defined."
It is a direct physical manifestation of the results of your efforts. It shows you that what you put in, or how much you are willing to put in, you will get back, and to the degree of which you put forth the effort. You can literally watch it happen before your very eyes which is intriguing both in the literal and the metaphorical sense of the principle.