So to summarize: You see someone frowning (or subconsciously note them making upset micro-expressions). You mimic their behavior. Your brain interprets your frowny reaction to mean that you are genuinely upset. You start to feel upset. (The same process works for smiling and other emotions/expressions, of course.)
This confirms the idea that in order to be happy you need to surround yourself with happy people, ones who can subliminally haul you up by your pouty lip when you're feeling down. As long as you don't get to them first, that is.