why one cries when happy???

Can anyone tell why one cries when happy,
Here's the thing: teeny-tiny almond-sized hypothalamus can't tell the difference between me being happy or sad or overwhelmed or stressed. Yours can't tell the difference, either. All it knows is that it's getting a strong neural signal from the amygdala, which registers our emotional reactions, and that it must, in turn, activate the autonomic nervous system.
The autonomic nervous system (the "involuntary" nervous system) is divided into two branches: sympathetic ("fight-or-flight") and parasympathetic ("rest-and-digest"). Acting via the hypothalamus (left), the sympathetic nervous system is designed to mobilize the body during times of stress. It's why our heart rate quickens, why we sweat, why we don't feel hungry. The parasympathetic nervous system, on the other hand, essentially calms us back down.

The parasympathetic nervous system does something funny, too. Connected to our lacrimal glands (better known as tear ducts), activation of parasympathetic receptors by the neurotransmitter acetylcholine results in tear production. (Fun fact: tears flow through canals that drain into the nose, which explains why your nose gets all gross and runny, too.