The possibilities are endless, so the cliche runs

I had an idea a few weeks ago that it'd be an interesting social experiment to spend a week approaching people that I've never met, and acting like we're really old friends. It relates somewhat to a book I read sometime ago, that suggests that people will generally relate to you as you relate to them. This could be used to explain the idea of Christmas 'spirit', or Christmas 'cheer'. Once an idea like this is established, it seems that it's contagious, but it relies on the agency of goodwill among at least a proportion of the population to keep it going year-on-year. In today's climate of mutual suspicion, don't forget the value of going out of your way to be pleasant, as the knock-on effects may be substantial.