A collection of passages that show our willingness to read anything about the brain as long as it comes from an "expert" and purports to explain something.
From wiseGEEK, Why yawning is contagious:
Most people, when they see someone else yawn, quickly feel the urge to yawn as well. Between 40 and 60% of people automatically find yawning contagious and yawn themselves. The standard answers from scientists as to why people find yawning contagious used to be that, although it was clearly a real phenomena, there was no obvious reason for it. Research conducted in 2005 by Finnish scientists, however, may point to certain parts of the brain as being responsible.
This does not explain specifically why people find yawing contagious, but it does suggest that there are brain sections responsible for a person’s perception of a yawn.