A man without a smiling face must not open a shop is a commonly-quoted Chinese proverb especially in relation to sales.

It also has definite impact on relationship-building. Being nice makes a difference. So is listening (letting the other do most of the talking), being empathic (able to see the other point of view), and respecting and honouring the other's opinions and thoughts. A smile is the opening act to these positive attitudes for relationship-building.

Dale Carnegie, in his book How to Win Friends and Influence People, listed many anecdotes to illustrate the powerful effects of these techniques (when applied genuinely of course).

The opposite traits are: needing to have the last word, needing to be right, needing to dominate, needing to be the focus of attention, and needing to have your say first, and needing to criticise or tell others they are wrong (even when they are actually wrong).