Why do people make others yawn when they yawn?

No definite answer. Contagious yawning has been known since at least the 16th century, but there has yet to be an explanation that stood up under testing. The best was that people have a grouping of "mirror neurons" in their brains that do imitative behaviors, but recent studies show that these neurons aren't activated during contagious yawning. It remains somewhat of a mystery.