We hear all the time how important exercise is and the amazing health benefits that it has on our bodies; What people tend to overlook or even forget to mention is the self control and discipline that's acquired after you actually start to see a physical change in your physique.
Self Control or self-discipline is a very important aspect of achieving and maintaining your desired physique, but it also plays a very big part of other areas of life as well.
When you become a person who constantly works-out, or what some people would call a "Gym Addict" there are some traits that you've subconciously developed along the journey of becoming a "Gym Addict" which would not at all be considered bad, depending on which spectrum you're looking at them from but from a fitness enthusiast perspective, it's what TONY THE TIGER would Call "GRrrr8" 
Without even realizing, you've acquire a great deal of discipline because obviously you've managed to make some decisions which would have caused you to put aside what you may have desired in that moment, to seek after a bigger more rewarding goal. which is why you are in the shape that you're in now
You applied self-discipline in at least two areas in your life(eating Habits & Physical Activity) in order to achieve your desired body goals.
Self-discipline is indeed the byproduct of each individual's journey to fitness.