The sentiments seem to be messing with sim's moodlets. My sim was disappointed during snowboarding on an easy slope because he wasn't "good at it". That's a weird type of thinking because he was a level 1 snowboarder and had only gone 2x to the slopes. So I don't think that it is accurate that they're randomly sad. They're also triggered if another sim (Especially one in their household that they have sentiments with) are sad. I had my child sim sad because his dad was sad about the disappointing snowboard experience. It made no sense, it affected certain actions that I wanted them to do, and it made them want to cancel certain actions I wanted them to do in order to "cry it out" because of the sad moodlet.