When someone else gets worked up, you're more likely to get worked up too, and if you don't want to be worked up, you don't want that person to be worked up. So, a lot of times when people say 'don't be sad' or 'don't be angry,' really what they're saying is 'I don't want to deal with you being angry or sad, and I don't want to feel that way, so I want you to calm down.'