It's impossible to point to any one thing and say "That caused it," but there were certainly a few contributing factors. For starters, it runs in my family. Aside from whatever physiological predisposition that may translate to, there was just a culture of drinking established by my parents.
Unlike many alcoholics, I never started early or hard. I could have counted the number of drinks I had before turning 21 on one hand. And even after that fateful birthday it just didn't interest me as much as the incredibly gorgeous (and abstinent) college athlete I had fallen for. But as that relationship flourished, stagnated, and eventually died among the continually ratcheting stress of a technical education... I started to rely on it more and more as a way to remove myself from the noise and emotion inside me.
When I graduated and landed a sweet, well-paid job almost immediately. Unfortunately it was also a brand new level of stress compounded by a horrible workplace environment and shitty boss. The bar downstairs became my regular place, and it soon became a habit to get a couple strong, cheap drinks to commiserate with my co-workers before stumbling home to my studio apartment for a nightcap.
I was pretty destroyed after the failure of my college romance, it definitely hindered my ability to get out and date effectively. I went on a lot of dates though, I was really good looking. I would usually have something to drink before any dates just to loosen up and in the space of a few years there were actually a couple fairly decent relationships.
Eventually I found a slightly less stressful workplace, but then blew any hope of relaxation to shit by committing myself to volunteering to lead a national student organization. In a general sense I care deeply for the organization's cause, but the people I work for are the most incompetent, petty, vile and entitled folks I could ever imagine existing. So, I continue to drink. Because I'm no quitter.
In terms of volume, I go through phases. Back in the day at that first job I could easily go through two or three liters of liquor (Vodka and Gin are my jams) a week. It fucked up my GI tract for sure, I threw up a lot. Every once in a while I'd be able to taper down to maybe 2-3 drinks a day, with less hard liquor and more beer/wine. For the past three years I've been at a level of about 1.5-2 bottles of wine per day, and only very recently (2-3 months) have taken it down to 3-4 drinks a week. If alcoholism weren't so humiliating, I'd celebrate my accomplishment more openly.
I've gained a lot of weight - about 40 lbs since my glory days in college. I can't stand the sight of myself anymore so I just don't look in mirrors or reflections. I find myself revolting to look at. I've also developed acute anxiety and depression, both of which are much worse after I drink, but both of which are also held at bay (if only momentarily) by drinking.
The one thing that is wonderful is that I've been in an awesome relationship with my current partner for almost five years. My partner doesn't really know the extent of my struggle, because while they don't have any alcoholic tendencies of their own, they grew up in an alcoholic household and developed very strong negative judgement toward alcoholics. We've talked about it a little in the past, acknowledged that there's a problem... but that's about the extent of it. I resent the judgement, and my partner resents the perceived lack of desire to change. I've never been violent when I drink, but I can tell it harms the relationship when I slip up.
So I guess those are pretty much the basics of it.