Let's look at loop 3 first. The more people who are taking drugs the more peer pressure there is to take drugs and then even more people take drugs.
And loop 2: As drug use increases, rules get stricter; as rules get stricter, more people rebel through drugs; more people use drugs.
And loop 1: The more drug taking, the more serious users; users need money to buy drugs; more people turn to dealing; the more dealers, the easier it is go get drugs; the easier it is to get them, the more folks will use.
All three reinforcing loops describe mechanisms that tend to increase drug use.