Humans are built as social beings. You find this in the way we react, generally, when we are around other people. Most times we tend to mimic the actions of others, and this happens intuitively for the most part. Hence there is a general saying that some of these actions are "contagious".

We yawn when we see someone yawn. We, at least, tear-up when we see someone cry. We laugh when we see someone else laugh.

But, of all these contagious actions, I consider the smile the most contagious.
There's something about a smile that brings calmness, or makes you comfortable or sends warmth to your soul.

I know, there are different kinds of smile that convey different messages, but for me a simple candid smile that expresses joy is what makes me smile. 😏🙂