Two different reasons (which are given much more rarely)

By contrast, there is one motive for drug use which, though rarely mentioned in policy debate: pleasure. It is pleasure which is essentially the defining quality of all illicit drugs: those drugs which are abused have pleasurable effects, and those which have no pleasurable effects are not abused.

A second unacknowledged motive for drug use does not even have a recognised name, but may perhaps be referred to as ‘consumer exoticism’. It is this which presents the most credible answer to the question of why the prevalence of drug use has increased so greatly over the last thirty years or so, despite the billions spent on its prevention. Most people in the Britain of the 1950s would have been deeply suspicious if offered an avocado; the same people now happily buy them from supermarkets. As consumer culture encourages adventurousness in our choice of food, clothes, holidays – it becomes ever more anomalous that we are asked to accept on faith, and against much of the cultural evidence, that alcohol is the only drug in which we should be interested.

Citation:

http://www.tdpf.org.uk/Policy_PolicyBriefings_WhyDoPeopleTakeDrugs.htm