I think this question would be best answered in two parts; the first section dealing with general reasons why people do drugs, while the second part would explain my personal reasons for drug use.
Humans have been ingesting mind-altering substances since the beginning of recorded history. Man has used drugs to enhance is connection to his god, and spirit, as well as using them for socializing and relaxed inhibitions. I think the latter is the main force that has propelled drugs throughout the centuries. Undoubtedly, civilizations have been constructed in fashions that are in many ways contradictory to human nature. In our own modern era, we have become a society ruled by rigid social standards, which guide us in how to speak, dress, and act. Wild, uninhibited behavior is frowned upon, even though it is solely in the free, unrestricted pursuit of pleasure that we find happiness. And this is why drugs continue to be so popular. They release us from the social mores that Western culture has ingrained in our minds since our inception. When people are inebriated, a whole new spectrum of socially acceptable acts is available to them. Suddenly, unabashed fornication, wild animalistic behavior, undiluted honesty, and even love, can now be openly expressed. Drugs affect much more than just neurotransmitters in the brain; they alter culture itself; they transform the standards of society from being diametrically opposed to human nature, to being in perfect harmony. This is the reason for humanity's obsession with drugs. Certainly, there are more reasons for drug use than just escaping moral standards. Some use them to diminish pain, others to simply relax and forget about their day-to-day problems. But, regardless of the reason, the central theme remains the same: Man cannot change his world, so instead he changes his mind.
As for myself, I use drugs for a variety of reasons. Sometimes I use drugs lighten my mood at party, or sometimes I use them to relax at home. On some occasions I use drugs for their innate pleasure; that ineffable feeling one gets when they perceive the world through new eyes. This is why I do drugs.
In addition to my response to the question, I would like to propose my own. Why do we continue to tolerate a society that prevents us from achieving maximum satisfaction? After all, we are all society; it is not some autonomous entity that exists outside of us. The current society we live in continues to persist only because we allow it to. To change it, we merely need to make coherent and collective choice to do so. If we truly want to be happy, we first choose to be happy, and be ready to fight those who wish to stop us.