Addiction.

"Welcome to addiction. In all reality, it’s not explicitly selfish. You realize you’re killing yourself but you don’t have enough self worth to beat the craving-reward mechanism, usually because of an underlying depression that is related to, and fueled by, some source of guilt/shame. It quickly develops into a self-reinforcing spiral of self-loathing that drives you to your drug of choice as an escape, which you can’t resist because you just don’t have enough self-worth to muster the will to stop, and worse, you’re cognizant of that lack of self-worth. That only legitimizes the self-loathing in your head and further deepens the depression and isolation, because who in their right-fucking-mind would actually want to spend time with you? Anyone who does is obviously too selfless and decent to deserve the load of shit that is your worthless life, but-you’re-just-so-lonely-and-the-only-way-to-distract-yourself-is-to-do-the-one-thing-that-overwhelms-all-other-thought-processes-anyway… And that’s why you keep getting high and/or drunk."

- fromkentucky on Reddit.