The stuff that makes you cry is actually pyruvic acid.
It has been bread into onions because it enhances storage ability. Basically the darker the skin and harder the onion the higher percentage of pyruvic acid it contains.
It also makes the onion produce more rings and skins which also increases hardness and reduces loss of moisture in the onion post harvest.
There is a bit of a trend towards sweet / mild onions (huge in the US). They have been bred to reduce the percentage of pyruvic acid in the onions. They tend to be larger, with less rings (but obviously thicker) and lighter more papery skins.
Pyruvic content is increased by increasing available sulphur, copper and boron in the crop but can also be tweaked using irrigation and fertigation techniques and varieties.
Know its a bit off topic but though it might be of interest.
