I really don't get the reaction to incest in some countries

Hello. I really don't get why some countries don't allow incest. In Sweden, where I live, we have no law that prohibits incest. At least I don't think so. I know that some countries have laws against it, but I don't get why. I mean, as long as you live with the person and don't show it to others, who cares? I know I'm 15 and still don't know the reason why, but I'm asking here because I feel I might get a correct answer.