Why is yawning contagious?

Not: Contagious means that it is passed on from one person to another. Seeing somebody yawn may make you think about being sleepy, and have you yawning as well, but this is only suggestive and not compelling.

Yawns are contagious: There are interesting studies showing that when one person starts yawning, others do follow. In fact, not yawning in response to others can be a sign of abnormal social behavior. But yawning is not "contagious" in the medical sense, it's a social interaction, like smiling back at someone when they smile at you :-).