why do people change?

why cant people keep the same great qualities they have at one time possessed...i know that changing is some areas is good and necessary, and that life experiences shape us and we are constantly being molded...but why cant some of our pieces harden and stay as they are? especially when some of those pieces were better before than they are now...

friends are changing, i am changing, life is changing...sometimes its good; but other times i would just like it to stop