Sometimes the person who is always happy isn't so happy after all.
Some people are so good at downplaying their sadness that even they don't realize how much they do it. When you ask them how they are they will always say that they are good, even when they aren't. They exhaust themselves by plastering an energetic and carefree persona in the spaces that you watch them in because at least to you they can control how they appear. They can pretend to be the happy person they want to be when everyone is telling them how funny and bubbly they are all the time.