When we chop or slice an onion, we release a gas, the lachrymator agent propanethiol S-oxide. It wafts upward and chemically reacts with the water in our eyes to form sulfuric acid. Our body then reacts defensively by producing tears to expel the irritant. Sweet onions cause fewer tears than standard cooking onions. Their higher moisture and sugar content lessens the quantity of tear-causing gas that is formed. Some people are innately more susceptible to the tear-producing chemical reaction than others. However, the more frequently we cut onions, the greater our tolerance. That's why the "crier" is more apt to be a person who cuts onions only occasionally.
** Asian Recipes