My twopenneth (sorry this might be a bit rambly).
There was a program about alcohol recently that was very interesting. In one part of it they had given monkeys free access to alcohol or water. Can't remember the exact figures but it was something like 10% tried it once and never again, 80% went and had some occasionally, and the remaining 10% kept going back and back and back and wouldn't stop until something external stopped them (the alcohol was removed, they were too pissed to get any more).
I don't think it's a stretch to say that could be pretty similar to humans - when I look at people I know some dont' drink at all, or if they "have to" eg a wedding toast they obviously don't really want to, most people like a drink but don't tend to go overboard. Then you have the people who will just drink and drink and drink until they fall down/the pub shuts/they get arrested/whatever. You can spot them by the look in their eye when alcohol is mentioned - they look excited. Many of these people regain some self control but it's a fine line - if anyone is going to get pissed and end up in a right state it's them.
I really do believe that some people just respond to chemical stimulants in a massive way, that their pleasure centres light up like a christmas tree and they've got no "off switch".
So that's one thing.
The other thing is that drinking excessively and constantly making a complete twat of yourself can lead to feelings of self-loathing - the whole thing can become a vicious circle. Most people don't like themselves very much when they wake up with a steaming hangover and remember all sorts of godawful things they did the night before.
Of course they may be something in the background. But I think it is a mistake to assume that there must be.
So really I agree with LRD that saying that she must have been abused to behave like this may well be a sexist approach in itself. Because nice girls don't do this - she is a nice girl - so there must be something wrong.