"Alcohol reduces the functions of the behavioral inhibitory centers in the brain, Forbes reports. It also slows down how information is processed in the brain. When you see, hear, taste, or smell something, your brain processes this information and then tells you how to think or feel. Alcohol interferes with this process, making it harder for you to work out what you are feeling and also making you less likely to be able to really think through potential consequences."
Thus, the thought of "just one" can take hold of you, and you won't be thinking beyond the dopamine hit. It also impacts your decision making about the amount you should drink, further enhancing your chance of a quit relapse.
A quit is a terrible thing to lose. Please keep it your #1 priority; don't drink.