Why Is Yawning Contagious?

The brain operates best within a narrow range of temperatures, and like a car engine, it sometimes needs a way to cool down. To lower the brain’s thermostat, researchers say, the body takes in cooler air from its surroundings — prompting deep inhalation. Source

Humans, the primates with the most complex social networks, rely on the ability to share others' emotions to engage in successful social interactions. This phenomenon, known as empathy, relies on a perception-action mechanism. The involuntary re-enactment of an observed behavior may arise in the observer by recruiting neural mechanisms that, during the perception of an action or of a facial expression, activate shared representations. Contagious yawning, evoked by the yawn produced by a conspecific and widely demonstrated in human and non-human primates, also involves a similar action-perception mechanism. Source