Can anyone tell me why I'm sad, tears flow, cry, why do tears come out of my eyes?

Naturally, nature produces tears for many uses. 
        Since it contains moisture to the eyes , helps to prevent eye infections. , Used to wash irritants out of the eyes. 
          But the reason that is Tears flowed from those eyes. Because the part of the brain that controls our emotions is located right in the eye area and this part of the brain that helps to release and vent our suffering. to be happy which by nature.

        When we are in distress, we must release that suffering by using tears as a tool to drain the suffering from our minds. Depending on what happened, that person would feel sorry. how much pain Water will flow out. And so are we when we are grateful or happy. This kind of emotion will stimulate and cause tears to flow out as well.

    Because crying will make you feel more comfortable This is because the body has eliminated various chemicals. that occurs while suffering leaves the body with tears.

         And if we observe that tears will flow in each emotion, There will be different flow characteristics, such as tears from stressful emotions will flow from the end of the eye. And if very stressful, It will flow from the centre of the eye. If in a depressed mood, it will flow from the corner of the eye. And if you're happy, I'm so excited that tears will flow out of my eyes.   

Ref : http://club.sanook.com/