Ah, the elusive achoo — there's much mystery surrounding our sternutation reflex, a.k.a. the sneeze.
Sneezing protects our nasal passageways from foreign particles by forcing a 100 mph whoosh of air from the lungs, but the reflex demands more movement than that. When stimulated, the brain stem’s sneeze center orders muscle contraction from esophagus to sphincter. That includes the muscles controlling your eyelids. Some sneezers even shed a few tears.
No one knows why. It's possible the body has grown to associate protecting the nasal passageways with protecting the eye, or it may just be a result of our body’s wiring. Nor do scientists know why some people sneeze at bright lights, or why the reflex is pleasurable to some but not others. Whatever its workings, the sneeze has the distinction of being the only reflex that warrants a "bless you" from total strangers.