A relationship can be start with pleasant one smile, because smile denotes the symbol of peace and love. And smile also be a part of relationship. We see sometimes couples always talk about smile of their partner, 

Smile works as the attraction between peoples. But there are some peoples who always like to smile, so don’t confuse in that case, because in this situation eye contact works. But smile will be signal of a friendship

I am not talking about parent child relationship. I am talking about relationship between couples. Specially, smile is a signal when two unknown people come in contact of each other, but it is either be for known peoples. 

So, always smile and make relationships.