I can't drink alcohol for medical reasons. It interacts badly with one of my prescriptions. It's really weird how often I have to tell people about it. Nobody really pressures me after I say "medical reasons", of course... but I'm very familiar with that confusion people get.
For me it's basically the same reason I don't drink soda. And yet no one questions me when I have an iced tea instead of a soda, but not drinking alcohol requires an explanation.