duq...
...what Holly is saying in a somewhat jaded fashion, is that nobody knows and it doesn't matter. The theories around it are many but, again, nobody knows and it doesn't matter.
It is what it is, they are what they are, we don't cause it, we can't control it, and we can't cure it.
I'd suggest to you that you stop trying to understand alcoholism, and start working on understanding yourself in the context of the alcoholic(s) you find in your life. There are people out there who won't, for even a second, enable an alcoholic or stay with one for even a second as a significant other. They know something we don't know.
What that something is? That's what you might consider trying to understand. Alanon, ACOA, and getting counseling for codependency issues is a good place to start.
Take care, take what you want, and keep the rest.
Cyranoak