Cravings are one of the biggest obstacles in cutting down or quitting cannabis. They cause that urge to smoke, even when you've told yourself you're not going to. You may experience cravings as physical discomfort, agitation or constant thoughts about the drug.
They're a pain in the neck to deal with, but the good news is they pass if you can hang in there long enough. Over time you get fewer cravings and they tend to get weaker and last for a shorter time. But in the first few days or weeks, it sometimes seems like they're never going to go away and you'll go mad if you don't smoke. It can be useful to think of cravings being like a hungry cat that meows "Feed me, feeeeeed me!" at your door. If you feed the cat, it knows it's on to a good thing and it's guaranteed to come back. If you don't feed it, the cat might try harder to get your attention, but it will eventually lose interest and go try somewhere else.
Cravings are much the same - the more you give in to them (and they can seem very convincing), the more they come back, again and again and again... If you can just ride them out, they do pass and they do go away.