There are several sources of stress, but the one that usually frustrates people is slaves hating each other. If you select a slave and click Statistics, then it will show all relationships with that slave on the right. Group sex is the best way to resolve this. Otherwise, unclean mansion, lack of food, sleeping in farm or jail, nudity rule, pregnancy, toxicity, high lust without sex, too many people sleeping in communal rooms, and strict headgirl are sources of stress.