Why do we cry when we are happy?

to what you call cry is related to various systems of body. it is because something positive to focuus. when you realize that everything around you is nice, eg you are in meditation, around trees, under those conditions when you continuously get fosued you get passion feel excited around. then you want t hem the trees and the falls. the feeling that you need is expressed in form of tears. sometimes child cry,??? So it is all due to social feelings, internal feelings and your internal capacity to express that joy.

Its a defense mechanism, kind of like laughing at a joke or dancing to a song. Emotions are not just feelings in your body they are literally energy and one way to push energy out is by crying.