Happiness is a great feeling no doubt, I do not contest. Everybody wants to be happy and satisfied. But you see, life isn’t like that, it beats you down in a unique way for everyone. And hence sadness comes into play.
If it did not have any evolutionary advantage, it wouldn’t have stayed in place, so many years after the first animals feelings emerged. Sometimes it is a legit way to decrease any amount distress an individual might be facing and it might be able to help people move on from various situations.
And so when someone is sad or distressed from a relatively trivial things, they are genuinely distressed. They may not have lost someone or something dear to them but we must realize that stress is affected by the perception of an individual, it is relative from person to person. What may seem distressing to you may not seem so to another person and vice versa.
So, when they ask you to fake happiness for the sake of others because your reason for being sad does not seem as prominent to them, be selfish sometimes (not all the times) and ignore them. You deserve to have a healthy roller coaster ride of emotions even if people may not want you to.