I have wondered about this.
My idea is that people usually focus on building their mind or their body. The problem is that your body is there just to get your mind from place to place. So it works both ways. If you build your mind and your body is treated badly (ie eating junk, smoking, drinking, drugs) and the body dies then you mind isn't going to do you much good. And if you spend all your time lifting weights and working out, but never read a book then you have a nice body carrying around an empty head and what's the point?