Two reasons typically given for the use of drugs

The primary explanation for drug use, historically, has always been addiction. Users take drugs simply because they cannot do otherwise.
In recognition of this, a second explanation for drug use has now become a standard account of motive: peer pressure. Drug education programmes typically identify this as the initial reason for drug use, and tailor their message to counter it: Just Say No.

Citation:

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