I think that something in a person's mind just has to *click* before they will do anything. That's the way it was for me. For some reason, in the period of perhaps a week, I decided that I would replace my car with a bike as often as possible. I don't know what spurred me to do this. But, the mindset stuck, and I'm better for it now.
It's unfortunate though, that the click never happens for many people. I'm still waiting for it to happen to my wife.