There is a thread in Cari Forum about how to improve one's social skill. Quite a few of good suggestions have been proposed; one of them suggested to always keep yourself smiling at least.
I nodded. I couldn't agree more I guess.
There was once a couple of weeks I worked in Kumon Learning Center. While I was doing some markings, a little girl came to my front desk and asked softly, "老师，你一直在笑什么？ (Teacher, what are you keep smiling about?)".