we cry because of the fluctuation of energy. when you feel sad your body vibration shift drasticly and cause a instant reaction to your eyes glands. Emotions allows us to control the global frequency at wich our body vibrates to. Love is the highest frequency. Fear is the lowest
People usually cry when they are sad due to the emotional distress and the tears have a different chemical makeup than those for lubrication. Crying at such a situation usually acts as a sign of relief and comfort.