For me it’s driving while under the influence. If you couldn’t tell, I like me some ganja. However I have long since held the belief that it is utterly insane to drive while under the influence of most substances, with maybe nicotine and caffeine being the exception. All too often I see other stoners smoking and driving, which I simply can’t fathom. I’ve only operated a vehicle once under the influence and it was just to move a U-Haul around the block to a different parking spot, which was such a scary experience while high that I refuse to even consider getting behind the wheel again while high.
I’m not sure about the weekly limit, but I guess it has also to do with the absorbtion of nitrogen into your blood, which is why you make safety stops after going deep and why there are daily limits.
Depending on how deep and how long you dive, more nitrogen will get dissolved in your blood due to the increased pressure. It stays there until you get into lower pressure ( ascending to the surface). If you do that too fast, the nitrogen will build up bubbles. And bubbles in the bloodstream is really really bad, hindering the flow of blood. Doing safety stops in lower depths gives some nitrogen the time to leave your body through the lungs while breathing. Not diving for a certain time after your dive sessions will give time to release all additional nitrogen from the blood, preventing a builtup over each dive.