I found that as well. I'm one of the most happy people I know, even when I seem stressed or tired or angry. In fact, it's because I can get stressed or angry that I can also get happy. 100% expression really means all expressions. Just because you are happy, that doesn't mean you can't be pissed off too.
All feelings really come from within, but most people let it be caused by others, and then later blame them for what's going on. It's very hard to be happy when your emotional state is completely depending on outside situations. Like a weather vane, you'd twist and turn in the wind. That doesn't sound like fun.
The happiest people can find the happiness inside themselves, express it to everyone and eventually see the happiness that resonates inside every event, place and person.
I wouldn't say your very happy friend is pretending to be happy, I think she discovered the truth.