Why do people crack their knuckles?

Answer 1

It can be a sign of nervousness to crack joints, usually the neck, when cracking your toes its usually means that you're bored and knuckles seem to be a somewhat universal sign for a preparation for action. It's not always a physical activity like a sport or beginning to play music, it can even be getting ready to talk to someone in such a way that might be stressful or otherwise nerve-wracking like starting an argument as a sign of superior strength. Knuckle cracking doesn't cause any pain and can be pleasurable because it relieves stiffness in joints, by the way recent studies (cited here: http://www.scientificamerican.com/article.cfm?id=crack-research) argues against the link to Arthritis.

In summary it's mostly linked to stress.