He takes us down a metaphysical wormhole, when all she'd said was "How are you?" People use that phrase as a greeting, so they are really saying, "Hello, please respond!" The normal response is, "I'm fine, how about you?" There are some people who think you're asking "How are you?" sincerely, and tell you about their aches and pains. It could be worse.
This guy instead focused on the word "are," as a form of the verb "to be." The state of being is a deep subject, depending on how deep you want to go. The question was not "How are you possible?" or "What are you?" or "How do you exist?" When you get philosophical over a simple greeting, people will tend to avoid you in the future. This comic is from John McNamee at Pie Comic.