Life inevitably involves change.  As human beings, we resist change even those that we know are good for us.  Exercising, like any other facet of our lives, must involve change.  If you lift weights, for example, how much progress would you make if you only ever lifted those little pink 2 lb dumbbells.  Granted, you would get really awesome at lifting the two pounders, but what fun would that be?   Giving up the nice cushy couch in the evenings after work for a three mile run involves a sacrifice also.  That couch looks mighty good after a long hard day at work!

When you first start an exercise program, it is very likely that you were sore afterward.  A week or two later, however, you realized that the soreness was not as bad and you felt like one bad mother!  But, as with anything, you gradually added more distance or time and realized that the soreness came back as strong as before...until your body adapted.  Thus, the challenge of change involves pain.

Change without some pain or loss isn't really change at all.  Not that the change is not good (for example, switching out a favorite pair of running shoes for new ones with more cushioning....your feet may feel better, but it can be difficult to part with equipment that you really love!)  Going up in rank in martial arts is an awesome accomplishment, but now the challenge is that you have to get out of your comfort zone, learning more challenging techniques or kata that may be more difficult than anything encountered before.  With higher rank also comes more responsibility....a brown belt can't get by with making the same mistakes that a white belt can!  Change can be positive even though it may not feel that way at the beginning.

Change is everyday life.  Embrace change; don't reject it.  Pain that comes with change can be overcome.  As change is inevitable, so too is progress.

"Life is a series of natural and spontaneous changes. Don't resist them - that only creates sorrow. Let reality be reality. Let things flow naturally forward in whatever way they like. "
Lao Tzu