Have you ever wondered why when someone yawns, the other person follows suit? Yawning is caused due to low levels of oxygen in the air, which makes the body intake large amounts of oxygen. The reason why yawning is said to be contagious is because when we do so, we are actually activating our nervous system. This has something to do with the survival instinct we have inherited in the course of evolutionary past. In the primitive years, when someone would yawn, the whole group would become alert and get better at sensing danger.