As one who seeks to cite an evolutionary explanation where one seems appropriate, I come to the use of drugs.  Why would we find widespread drug use when the resultant perils are prevalent, serious, and somewhat likely?  I think humans use drugs recreationally to feel, first and foremost, even if there is high probability that negative feelings will be included.  Early humans were crafted in an environment where boredom and/or non-feeling surely were paucities.  Our environment is one where the thrill of a successful hunt, despair of imminent starvation, scramble for scarce resources, and the sheer fatigue of daily existence is essentially absent.  But our bodies haven't changed proportionately, thus, the need for an adequate emotion-inducer to satisfy that void.  Perhaps even the up and down nature of drug use mirrors deeper emotional fluctuations present in early times that were dependent on those now absent factors.

Drugs aren't the only way humans attempt to self-inject emotional experiences.  Horror movies satisfy the need for immediate fear (one would hope we could get over this one), music adds a flavor to mundane events (I think music woudn't have been as enjoyable to early humans), and athletics in all shapes and forms emulate the competition and exertion inherent in natural life while also stimulating the corresponding emotions.