D Reynolds of Dunedin asks :-
Why do we shed tears from our eyes when hurt or upset?
Gordon Sanderson, an opthalmologist at Otago University, responded.
This is a very good question and not an easy one to answer. If we begin with the reason why we have tears in our eyes at all, it is that our early ancestors were actually fish who lived in the sea. Then they left the sea many millions of years ago but their eyes still required a little bit of sea water to keep them working properly. So we evolved eyelids and a lacrimal system to produce salty tears.
You may find it interesting to compare the eyes of fish such as sharks with those of aquatic mammals such as dolphins. You will notice that dolphins have eyelids because they previously lived on land and have gone back into the sea whereas sharks never left the sea and do not have eyelids.
Tears are formed in glands called lacrimal glands. There are three main forms of tear stimulation: one is called basal stimulation which provides the normal tears for keeping our eyes moist and our corneas healthy throughout the day. The second is called reflex stimulation and is why we respond to things like dust or smoke in our eyes or even juice that squirts from an onion that has been cut. The third is the one that you referred to namely emotional stimulation. It is very hard to explain, but if you understand that the lacrmal gland, which produces most of our tears, is stimulated by nerves and these nerves are connected to various parts of our body including our eyes. This is why our eyes water when we get something in them. They are also connected to various parts of our brain including an area called the limbic system. This part of our brain is responsible for emotional behaviour. From here the impulse travels to another area of our brain called the pons and from here along nerves called autonomic fibres through various paths to the lacrimal gland.
So what all this means is if you are in a sufficiently emotional state, either sad or happy, it is possible to stimulate your lacrimal gland and start the crying process. You have asked a very good question and a lot of the books say "very little of this particular response is understood" so maybe this is an area for you to do some research on when you are older.