The question itself is interesting. Nobody asks a drinker why they are drinking. Pour a glass of wine at a dinner, and the question is what kind. Decline the glass, and the question is why.
This is not a complaint. It is just an observation about how default the drinking is in adult social life, and how visible the choice to not drink becomes by contrast. The question is going to come, in some form, from someone, at most events. It is worth knowing what you are going to say.
The short answer that works
The instinct is to explain. The instinct is wrong.
The short answer that works is some version of: "I'm taking a break." "Not tonight." "I quit a while back." "I'm driving." "I'm on a thing." Pick whichever is closest to the truth and easiest for you to say in a normal voice.
The reason the short answer works is that almost everyone who asks the question is not actually asking for a story. They are making conversation. They want a quick reply they can react to and then move on to the next thing. If you give them a short answer in a normal voice, they will move on. If you give them three sentences with a hedging tone, they will ask follow-ups you did not want to invite.
The short answer is also more honest. Whatever your real reason is, it is yours. You do not owe a justification to a person you are about to spend forty seconds with at a buffet line.
The longer answer for the people who actually want one
There is a small set of people who will ask the question because they are also asking it, of themselves, and your answer is part of how they think about their own. Spouses of friends. Coworkers who have been quietly cutting back. People in their early thirties who have noticed the pattern and want to talk to someone who is further down the path.
For those people, a longer answer makes sense. The longer answer is something like: "I stopped because I noticed I was sleeping badly and felt worse than I wanted to feel, and the math just stopped working. I have not really missed it."
That is the whole answer. It does not require recovery vocabulary. It does not require an origin story. It does not require you to claim to be cured of anything. It is just a description of what happened, in your own words, in a tone that is not seeking validation.
You will know who is in this category by how they listen. The people making conversation will already be looking past you for the next thing. The people who are asking because they are also asking will be still.
The version that is just "I just don't"
Eventually, you arrive at a version of the answer that is barely an answer.
"I just don't."
This is the late-stage answer. It tends to show up around month six or year one. By then the question has been asked enough times that the answer has compressed. By then the question has also become less interesting to you, in a way that comes across in your voice and that closes the conversation cleanly without you having to do anything else.
This is the answer that most longtime non-drinkers settle into. It is not defensive. It is not informative. It is just the truth. They just don't.
What changes about the question over time
The first three months, the question is going to come up at almost every event you attend, and you will find yourself with a small inventory of responses ranked by how much energy you have for the conversation. This is normal and it gets quieter.
By month four or five, the people in your regular orbit have learned your answer and stopped asking. The question now only comes from people you are meeting for the first time, which is a smaller surface area.
By year one, you will notice that the question itself has shifted. Earlier, the question was "why aren't you drinking tonight?" Later, it becomes "do you not drink?" The shift is small but real. The earlier version assumed a baseline and asked about a deviation. The later version treats not drinking as the description of you, and the only question is whether the description is accurate.
By year two, almost nobody asks. You have arrived at the point where not drinking is part of who you are in the same way that not smoking is, or not gambling, or not eating shellfish. It does not require explanation. It just is.
The thing that takes longest to learn
That the question is theirs, not yours.
When you stop drinking, especially in the first few months, you carry around the assumption that every time the question comes up, it is a referendum on your decision. The instinct is to defend, justify, qualify, and reassure. The instinct is exhausting.
The truth is that the question is almost always being asked by a person who is briefly curious or briefly performing a social motion. Their question is not a referendum. It is conversation. The right amount of energy to put into the answer is the amount of energy a normal piece of conversation gets, which is to say: not much.
The shorter the answer, the more the question goes away. The more the question goes away, the less it feels like a question at all.
Eventually, you will be at an event and someone will ask, and you will give a one-word answer and not remember it five minutes later. That is when you know the project is done, and the next part of your life is the rest of your life.