I told a friend after I returned from Russia that Russian smiles are made of gold - they don't let go of one without a good reason. But when I was introduced to a new person, they would often smile as they said здравствуйте. When I bought half a dozen books on Russian for Foreigners, the cashier smiled as she asked "Are you learning Russian?"

If a Russian smiles when you start speaking Russian to them, it is probably because you have a funny accent (which I do). If you go around smiling for no good reason like some Americans do, you may be thought to be insane.