There's a lot of biology involved that I don't understand the intricacies of, but basically the more time and at more depth you are underwater the more your body is dealing with changes in the density and makeup of your blood. You need time on the surface to normalize. For a similar reason you need to do safety stops to allow your body to adjust to the changes.

For normal recreation dives, it's pretty simple that people are limited to (IIRC) two dives/two hours per day as a general guideline. Once you get into deeper dives, using different breathing mixes, and other stuff people have to start doing a little bit of double checking to make sure they don't overdo it.

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.