Your brain never switches off. While you're ambling through life, it is constantly evaluating the expressions on other people's faces. And it takes only 0,1 seconds to register a happy or a surprised face, and a bit longer to see one that looks sad or fearful. First impressions are indeed important, if not always right. If you follow the principle that in 90% of cases you're more likely to get what you want if other people like you, best you get a smile on that dial and make it snappy :-)