Some of the most common types of behaviors that are the most contagious are: yawning, laughing, smiling, frowning, shivering, and risk-taking. Why do we mimic other behaviors that we see? Our brains are hardwired for social interaction and bonding. Mimicking the actions we see in those around us is a natural way that we empathize and gain a sense of how others are feeling.
Below is a video clip explaining in more detail how and why yawning is so contagious.