In reading the answer to Why do we yawn?, I was eager to find out why, then, are yawns contagious? It really seems to be true that if you watch someone yawn, within moments, you will yawn as well. It's a very curious thing.
— Right under your nose
Dear Right under your nose,
You are not the only curious observer of human behavior; scientists and others studying yawning seem to have reached a consensus that yawns are indeed contagious, or at least that people tend to yawn in groups (a single yawn by one person is usually followed by yawning from others). The "why" part is a little more controversial. Here are the leading theories:
The physiology theory
Yawning is a reflex triggered by the body's need for increased oxygen (as mentioned in Why do we yawn?). In a situation where one is not getting enough oxygen (think stuffy lecture hall), others are probably also feeling the lack of O2. Seeing others yawn is a reminder to one's body that its own cells are feeling deprived, similar to how watching someone else munch a candy bar can provoke a chocolate craving in one's belly.
The boredom theory
Yawning is a way of displaying to others or ourselves that we find something tedious or dull. Chances are that if one person finds an activity mind numbing (go back to that lecture hall again), others are sure to agree.
The evolutionary theory
Yawning is a behavior left over from our bygone caveperson days, when it served as some sort of social signal to others in our pack). When we yawn at others, they yawn back to return our long-forgotten message.
The social theory
Another theory is that yawning is a social and communicative function that indicates mild displeasure without demonstrating a fear of an immediate threat, such as in the case of hunger, mild stress and drowsiness. There have been studies that suggest that contagious yawning is part of an empathetic response to others. Remember that none of these theories have been proven and are still being studied, and probably require much more painstaking research, discussion, regression analysis, and um... linear, uh... graphs, and... stuff! YAAAAWWWNNN!!!