Pleasure and pain are part and parcel of our life. Blending of the emotions like happiness, sad, anger glorifies the life.
Tears have the primary function of cleaning and lubricating eyes. Tears shed down from eyes for different reasons. It may be reflex reaction like cutting onions or dust in eye; it may be during extreme emotions like grief, sadness or may be due to some underlying pathology.
It is a well-known fact that we cry when we laugh hard...But why??? Most probable explanation is laughing hard creates pressure on nasolacrimal ducts. So tears cannot flow into nasolacrimal duct as they usually do in normal conditions. At the same time eyelids also close. Hence tears cannot flow into eyelid. So tears come out. On the other hand it may be a homeostatic mechanism that brings body to a normal state after extreme excitement accompanying the laugh. Researcher William H. Frey, II found that emotional tears are rich in protein based hormones prolactin, adrenocorticotropic hormone, and leucine enkephalin. Dr. Frey had stated that crying releases chemicals that the body produced in response to stress. Laughter and tears are helpful to tackle stress as both of them counteract the effects of cortisol and adrenaline.
Always remember the fact - a good cry during extreme pain or pleasure lightens your heart and bring peace to your mind !!!