Mirror neurons fire both when one acts and when one observes the same action performed by another.
These mirror neurons may be the neural signals for imitation and the emotional basis for empathy. Thus we are naturally born to share our moods such as happiness and to have empathy for each other.
What are mirror neurons?
As Dr Seppala: http://www.emmaseppala.com/the-science-of-compassion/?utm_content=bufferc5cae&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer , shared, it is likely that our mirror neurons are evolving as we become more compassionate toward each other.