When I'm getting drunk and really enjoying it, the answer seems to be: because I enjoy it!
That makes total sense.
In my drinking, I start to notice some strange things, however.
Firstly, I would drink a ton, to the point of giddy excitement, but I would then drink way beyond that to the point of aggression, darkness, and stupor. I was not drinking each additional drink because I 'wanted to' in the ordinary sense of identify a need, assessing the options for fulfilling the need, weighing the pros and cons, and then deciding rationally another drink would be good. There was no decision-making process: I was simply yielding to an overpowering impulse.
Secondly, I would drink, and drink a ton, even if I knew it would jeopardise the current situation (because I would likely behave badly) or my ability to function the next day, and even if I was having a horrible time and knew a drink would not help. It was situations like this that laid bare the truth: I was drinking to excess because that is the way I am built.
The chapter 'The Doctor's Opinion' in the book 'Alcoholics Anonymous' explains this in some depth, but the basic gist is this:
I was neither dumb nor mad, and my excessive drinking persisted through good circumstances and bad, through good emotions and bad. The only satisfactory explanation for craving past the first drink was that there was some process within me condemning me to drink compulsively that lay beyond the exercise of my will. The book calls this a 'physical craving', but any action I take is also mediated through my mind, so it always 'feels' like my mind is telling me to drink.
However, the craving lies inaccessible beyond processes of reason, hence the theory it is physical in origin.
In any case, it does no good to get too tied up in theory: I think of it as a 'constitutional' phenomenon. When I drink, I drink too much, because that's how I'm built. If that's the case, I need to never have the first drink, because, if I do, I might never stop.