Ascending Norms

The intricacies of elevator-specific social norms are one of the first pieces of corporate life most people pick up. When it's ok to spark up banter, to push the button or request a floor, how to file out, etc. There aren't hard and fast rules, but there's certainly a general structure. There's one element of this structure that mystifies me. Just about everyone tends to abide by it, but I just can't make heads nor tails of it. Why is yawning an invitation to commentary?

I'm a pretty yawn-prone individual, especially in moments of inactivity (such as, standing still in a small, vertically moving box). Everytime a yawn creeps to the surface while I'm in the elevator, it for some reason forces my co-riders to discuss how it's too early, or too late, or too sleepy, or that time of day, or the likelihood of me having a late night, or how busy my work load is, or how much I would rather be at home in bed than in the elevator, or the effects of the weather on my slow and sleepy intake of air. What gives? If I see someone scratching their shoulder, I don't start pondering their most recent exposure to poison ivy, or discussing the merrits of moisturizing after a shower. I don't tell someone who's coughing that it's too deep and they need to see a doctor. It's not necessarily offensive to comment on my yawning, but everyone does it. I'm just a little confused. Why? Why is it so captivating to see a 24 year old less than perky as she heads to or from her office? Isn't our whole damn nation sleep deprived? Do I look particularly shocking when yawning? I just can't wrap my brain around it.
That, and the fact that while I can yawn with about 99% certainty of a subsequent discussion, I can still sneeze with only about a 50% chance of getting a "bless you" or some similar nicety. This is a very fractured observation, but I just want to know if anyone else experiences this, or has some idea of why it happens. Thoughts?