Because someone who is pushing alcohol on you is not thinking about you. They are thinking about about how they want you to have alcohol, which has nothing to

Because someone who is pushing alcohol on you is not thinking about you. They are thinking about about how they want you to have alcohol, which has nothing to do with you. You do not owe them an explanation about why their plan for you doesn't work for you. To look at it another way, it easier to explain things to someone who is trying to listen.