Try not to smile back and you will understand ... It happened with me when I was in university. Once I took 80% of my exams bad. I was not ok at all. I had a very bad mood. When I was coming from the university and met some of my school friends. He smiled me but I did not smile him back. I remember very well that I thought: "Why do you smile me. We are not so close friends." I just said hello then we had short conversation and then goodbye. In several days I met my best friend from school. Frankly speaking he is my best friend now. He smiled me he had not seen me for quite long time but I tried not to smile him back. It was impossible not to smile him back. In a moment I recollected lots of things which we did together. I remembered many events. And I even did not notice now there was a smile on my month. If you are not sure, who is he or she for you. Just try not to smile back.