Why do eyes water in response to particular emotions? It is common for people to experience very moving situations (sad movies, etc) and have a physical reaction to it (such as crying). I am curious about how emotions can physically lead to the act of crying. Why/how does this happen? What is the mechanism?

When we are sad, our emotions are in an overdrive and the parasympathetic nervous system triggers lacrimation i.e. tears, and we cry.