Why random strangers smile at you?

People sometimes smile to others to make them feel comfortable or at peace. This is one reason why random strangers sometimes smile to kids.

In some cities, people smile to strangers as a part of their culture. In such a case, strangers can smile to you as a result of the habit they learned.

People can smile to others to show that they are nice and friendly. This is one reason why some people smile to others inside elevators.

Extroverts find it easier to connect with strangers. Extroverts are thus more likely to smile to strangers than introverts. (See Why are some people Extroverts?)

People sometimes smile to strangers in order to start a conversation with them. In such a case, a certain level of interest might be present.

People who are happy are more likely to smile at others than people who are not. If a person is in a good mood then they might smile to random strangers.

Some people are overly friendly. Those people might spread smiles as they interact with others.

If a person wants to ask for help such as asking for directions then they might smile to you first. If those people got a smile back then they might proceed to ask for the favor.

Some people smile at random strangers because they like socializing. If those people get a smile back then they will start a conversation.

When a person feels confident about themselves, they might feel like smiling to others. Smiling to people in such a case is one way of being humble.

A person could smile to you if they believe you are attractive. In such a case, those people might be hoping to get to know you. (See What makes a person attractive?)