It’s surely genes, at least in part. Genes + health and drinking etiquette (keeping hydrated, eating well before, during and after, etc) all contribute. In my experience, genes play a huge part in it. I have friends, that eat like shit and have bad health in general, that are able to drink more than anyone else yet don’t really get hung over. Some of them only drink a couple of times a year, so I’m sure it’s not just tolerance.