Lelly have rised my curiosity about "why the yawn is so contagious?". And, obviously, Wikipedia has the answer:
The yawn reflex is often described as contagious: if one person yawns, this will cause another person to "sympathetically" yawn. The proximate cause for contagious yawning may lie with mirror neurons, i.e. neurons in the frontal cortex of certain vertebrates, which upon being exposed to a stimulus from conspecific (same species) and occasionally interspecific organisms, activates the same regions in the brain. Mirror neurons have been proposed as a driving force for imitation which lies at the root of much human learning, e.g. language acquisition. Yawning may be an offshoot of the same imitative impulse. At a distal level (in terms of evolutionary advantage), yawning might be a herd instinct. Other theories suggest that the yawn serves to synchronize mood behavior among gregarious animals, similar to the howling of the wolf pack during a full moon. It signals tiredness to other members of the group in order to synchronize sleeping patterns and periods of activity. It can serve as a warning in displaying large, canine teeth. This phenomenon has been observed among various primates. The threat gesture is a way of maintaining order in the primates' social structure. The contagion of yawning is interspecific, for example a human yawning in front of a pet dog can incite the dog to yawn as well. Oddly, sometimes sympathetic yawning may be caused by simply looking at a picture of a person or animal yawning, or even seeing the word yawn. ...
Concluding: there is no sure answer to this question. Only suppositions and theories. How many mistery about [en:Human_Being|human being] still exist?
More details about [en:Yawn].