In my early 20's I was very curious about what went into our foods that we ate. I was absolutely shocked. As well I have also been a very lean guy with a crazy fast metabolism. After reading a few issues of Muscle and Fitness and Flex magazines, I decided to start eating very healthy and working out. Boy did I have a ton to learn! I ended up doing way too much cardio, the circuit really fast etc. Got down to a 3% body fat and started having some serious health issues. So I ended up leaving the gym for sometime and mostly tried to eat healthy.
Picked it back up in my 30"s and stopped after 2 years, and now in my 40's, I am actually going to achieve my goals this time, as with time has come wisdom.
And as of 2015, I have gone back to the gym for myself, and my Spouse, as she has been whining about her body for a couple of years, and I finally convinced her to either put up, or shut up. She is doing well now, not so much in the beginning. And we even found a protein for her in her favourite flavor, Birthaday Cake!! so it motivates her through the gym :)

Simply put it makes me feel good. It makes me feel good to accomplish what I do at the gym, as well as setting goals in the gym and achieving them. Wether its a few more pounds on the bench press, or more situps, or even the dreaded treadmill. Its nice to set goals and achieve them in the gym, as the rest of life isn't as easy or doesn't go as we planned. Also when I look in the mirror I have a greater pride of my image that I see, as I did it. I built it, I set a goal and achieved it. And last but not least, as I make progress, people notice, and well its very flattering, and inside makes me feel good.