I think it's quite healthy to ask why, or how. After all, if we don't understand we can't overcome and move on.
One of the most important things I learned by doing research into alcoholism was that there is no reasoning with a drunk. You cannot discuss things with them and expect sane replies. That definitely mimics a mental disorder, I just don't know which one.
And to the many many spouses and partners of alcoholics and addicts who became that way long after the relationship started - no, it's not you and it wasn't some failing on your part in your choice of a partner. You didn't cause it, and you can't control it.