Among humans, a smile is often an expression of happiness. To get inspired, have a look at these photos, watch this 1 minute video, and/or glance through these top 10 reasons to smile.
Among the 10 reasons mentioned, I find, not least, reason # 3 "Smiling is contagious" quite interesting. As the author writes, "when someone is smiling they lighten up the room". In other words, people who smile influence the project/company culture in a positive way.
Do you have some examples of how smiling affects the culture of the project/company you work for?