it is because some of the students have a "引" to try it,and when one of the best friend are try to eat it they will try too and so on and so .......Also so people who are salling the bad thing are telling them when they few any thing is unhappy or sad , they can try the "thing" that they buy .So every time when they have problem they will eat bad thing, so every people is try to use the bad thing to have快感,and to forget the things that they don't want to remenber.