The martial arts are based upon understanding, hard work and a total comprehension of skills. Power training and the use of force are easy, but total comprehension of all of the skills of the martial arts is very difficult to achieve.
A lot of people change for good. Some people just fall off. Just trying to progress in anything, no matter what you're doing, I feel like any progression you make... some people aren't gonna be around you that were around you.
We are in a survival mentality, and that's hard-wired into our humanity, because we are the winners of an evolutionary struggle of millions and millions and millions of years.