@NOEngineer:
I also have a strong, repetitive desire to drink water, eat food, and go to the bathroom due to certain overwhelming signals from my body. Sorry for the sarcasm, but there seems to be an assumption inherent in much of this site that the desire to crack joints is somehow false or contrary to what is healthy.
Here are some things to consider:
Perhaps pressure inside the joint is bad. I work in the chemical industry, and relieving excess pressure from tanks and vessels is a requirement to prevent damage to them. An imperfect analogy, I know…
Perhaps the pressure is caused by some failure of the joint membranes that allows more gasses to migrate into the joint, and/or less gasses to migrate out. I would assume that the joint is alive, and therefore needs oxygen to be supplied and CO2 to be removed.
Alternatively, maybe there is something in the cracker's system that generates more gas inside the joint. Decomposition or other reactions of chemicals can give off gases.
On the other hand, maybe everyone has the same amount of dissolved gas, but different nerve sensitivity to the resulting pressure. Just as some people have sensitivity to noise or light levels that don't bother most humans, maybe crackers have extra sensitivity to pressure. I doubt this is a cause, since my body involuntarily cracks when I stretch, walk, etc. But, maybe I've just loosened up every joint in my body over the years because I have cracked them on purpose so many times due to heightened sensitivity....
Thank you for sharing your theory, NOEngineer! Something to ponder on, for sure!