This is a bad question, as those are two completely separate things. "Appearing" happy is a facade, a mask. Positive thinking is an internal process where you always try to see the good side of any situation, but it doesn't mean that you can't be fazed.
In my experience, one who goes out of their way to appear happy all the time, usually drop their mask explosively, suddenly, and sometimes violently under the right (or wrong) circumstances. Keeping your emotions bottled up is never, ever a good thing.