Whilst this gesture is frequently understood to be a happy, friendly greeting or emotion, it actually varies within different cultures. In Japan and other parts of Asia, someone may actually smile when they are miserable, are embarrassed or are annoyed. A smile may be reserved for a greeting for a friend and not necessarily be there for Joe Public.
Many Russians consider smiling at strangers to be abnormal behaviour and will view this with suspicion. .
Film: Bob-sled Gold for Britain in 1964