THC acts on the reward system in the brain. If this happens often, the brain adjusts: it becomes less sensitive. It then needs more for the same effect. Experts call this tolerance.
Tolerance is a sign that the body has got used to cannabis. It is one of the criteria experts use to recognise dependence. The Federal Ministry of Health explicitly describes dependence syndrome as including the development of tolerance and withdrawal symptoms. Anyone who takes more to reach the usual effect also increases the risks, because they rise with the amount and the THC content.
Often something else comes on top: the high is followed by restlessness, irritability or discomfort. Over time, people then tend to use to get rid of this discomfort rather than for the high.
Tolerance goes down when use stops. A break is therefore also a test: how easy do you find it?
Remember: If you need more for the same effect, that is a warning sign and a good reason to take a break.
More on this: How dependence develops · Taking a break