There are a two main theories I'm familiar with to explain why yawning is "contagious."
1. The most commonly cited theory is that people yawn to get more oxygen into their brains. When we see a person yawn, it triggers a yawn in us, to ensure that we have as much oxygen as we need.
Studies have shown however, that even in a setting of pure oxygen the yawning response is still triggered. This suggests that the response is not because we actually need oxygen but b/c we think we do. (The tern "thinking" is really a misnomer since the resonse seems to be autonomic).
2. Yawning may have been an extremely adaptive early form of communication, either as a means of alerting the group that it was time for bed, or more likely, that it was time for some communal energetic task such as hunting. The yawning synchronized the group and got their energy at the same level.
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