Hi Tammy,
I've read your posts since you first got sober and was impressed (still am) at how you just took the leap with the Big Plan and ran with it. I'm just ten days in now, and I'd like to say something self-deprecating about that little amount of time, but realize that time doesn't matter at this point because I am now a non-drinker. All disclaimers aside, we're very close in our recent decisions to quit after a long period of drinking, and I want to offer my support.
For what it's worth, I spent years searching for the "why." Wise people here suggested that I forget about the why and just get on with it. I couldn't, I was stuck. Then, after about 5 days sober, after having made the decision to stop forever, I had a hard think about how I would explain alcohol dependence to someone who wasn't an addict. And I hit on, "It's like if you decided you could never have sex again." This flipped the switch for me! I could do that. I wouldn't necessarily be happy about it and I would miss it like hell in the short term, but long periods of abstinence have proven that the memory fades. Not sure if that's helpful, but I thought I'd throw it out there.
I think that the reason "you" can't come up with a why for drinking might be one of two things (or maybe a million, but I can only think of two at the moment). "You" are doing the thinking, but you are not in charge. You are feeling the Beast doing its thing and the anxiety and related physical sensations that generates are overwhelming. And/Or you are having a hard time (as I did) breaking it down to it's simplest form. As GT points out, there is only one reason to drink. I believe now the rest of it is rationalization, and maybe in part a perverse enjoyment of playing the game that MesaMan wrote about. In retrospect, I certainly did that.
Congratulations on making the decision to take action. You've done this before and you can definitely do it again with lasting success.