Sometimes some persons get more tears during onion chopping and some persons get less....what is the reason behind it ?..?

Dear student,
Please find below the answer:

The lacrimal glands situated at the upper side of the eye secrete tears that help in lubricating the eyeball and prevent it from drying. Some people have dry eyes or less moisture in eyes as compared to others, hence they produce less tears. 

This is why some people (with more dry eyes) get less tears while chopping onions while others (with proper moisture in eyes and highly active lacrimal glands) get more tears.

Hope this clears your doubt.
Regards.