It would appear some are more sensitive than others? I can usually chop (i.e. completely dice up) half an onion, sometimes the whole, before I start tearing up. Certainly just peeling and quartering the thing doesn't cause any effects. Perhaps because I wear glasses?
However, once I start crying, I can't stop, even if I step away and take a short break. I guess tears already in the eyes make it easier for the fumes to get in and irritate the eyes further? In which case, I guess those who are "resistant" to it probably have dryer eyes to begin with, while the "sensitives" probably have moist eyes?