Emotions are chemical levels in your brain and your body is constantly trying to maintain equilibrium. If one emotion sky rockets, that chemical becomes flagged and signals the tear duct to open as an exit to release that emotion packaged neatly within a tear. It’s why we feel more stable after crying, as if whatever emotion we were feeling had been released and we were refreshed. This is also why tears from different emotions look different under an electron microscope. They’re literally made up of different things.
According to Joseph Stromberg of the Smithsonian’s College of Arts and Sciences, happy tears are structurally different than sad tears than angry tears than overwhelmed tears etc. Different types of tears have distinct molecules. Emotional tears have protein-based hormones including the neurotransmitter leucine enkephalin, which is a natural painkiller that is released when we are stressed.