I started weightlifting a bit in high school, but got serious for the first couple of years of college with the same intent as everything in my life up to that point. To improve my athleticism for baseball. Life, injuries, and other health issues would derail me for the next 10 years, and I finally was able to restart May '13.
The first and foremost reason is that I love being athletic, and that allows me to improve in all the sports I participate in. It gives me energy and confidence that I otherwise don't have. It's a challenge that allows me to push my body to its current limits, and by doing so expand those limits so that I can go further tomorrow. I also find my time training to be extremely therapeutic. I go in with a detailed plan, and for the time it takes to execute that plan I don't have to think about anything except executing that plan. In a world where we are constantly being pulled in so many different directions it is really nice to be able to be completely selfish for a time and just turn the brain off and tune everything out.