Man there are lots and lots of reasons. One is that by doing it daily my body has become accustomed to it, I don't feel right if I don't get my daily endorphin fix. Two, I enjoy the feeling of health, vitality, and energy that it gives me. Three, I know the work that I do today might be the only thing that saves my life one day. Four, in guys who work out 5 days a week, there chances of dying from anything are much lower then sedentary people, its called all cause mortality, it also reduces your risk of cancer. The stronger you are in your youth the better chance you have of being able to live independently when you're old. The alternative being to lie in a nursing home some where where people have to change your diaper and you lie there waiting to die for 10 or 20 years...no thank you.
Now I know that I am probably going to get flamed like crazy for this, but some of it is vanity. I love walking into a bar and watching them fight over me like a pack of wild dogs. haha. It's nice to be able to go out and have people notice you, its nice that they notice all the hard work you put into looking great. I really think this sets you apart from most others. Lets face it, it takes a lot of discipline, commitment, and ass busting work to look great.
Last, a commitment to staying active and fit, or getting there, has a way of effecting every other area of your life. You get in shape and you hold your head a little higher, there's a little more spring in your step, you have energy, confidence, vitality, and life pumping through your veins. That's the kind of thing that seeps over and improves many, many aspects of your life, I would say damn near everything, from your professional/work life to your personal relationships, etc. Man this is good stuff, if you just can discipline yourself into sticking with it for six months I guarantee you will see this start to happen, and it will become a lifestyle instead of something you have to do. I have yet to see anything else that can have as much of a profound effect on peoples life, and that is why I train myself and I train others.