Friendship is tied by itself. If you have lots in common it occurs by itself, but if your interests don't match what you would not do friendship is not. Only very difficult to be just friends with a beautiful girl. You will necessarily have deeper feelings and it will interfere with the friendship.
smile,
with a smile to the person we meet, shows that we are happy to meet him. A sincere smile means that we're happy. And pleasure is one form of positive emotion that we can "transmit" to others. This pleasant condition allows us to interact better