That is one of the great things of life. To find out what to live for. It may be hard, but you have to find it for yourself, because not everyones is the same. Are you just going to sit there? Or are you actually going to have an impact on mankind's future?
All up to you. I work out, but not for most reasons most meat heads do. They do it for chicks, money, fame, etc etc. I do it because it is a challenge, because it is a productive way of extorting your anger and other strong emotions to fine tune your body. Sure, it's cool to have a nice body, but that's just a side dish from the main meal. In my mind, it's sort of a "master your body, then master the world" type of attitude, because let's face it, having a big body intimidates (most) people.
I was a fat little turd when I was younger, and then one day, I thought to myself, "What do I have to live for? Pop tarts?" So I started to work out. Now I have a well built body. That ALONE is proof enough that you can do it, (whatever your goal in life is.) Obviously, that was not my goal, just a brick on the road of the story of my life. So go forth, and do what it takes to fulfill your dream. Something you've always wanted to do. Something that, when you die, you can say, "You know what? I lived a damn good life." This is your story. And yours alone. I wish you luck.