Well, science says crying is more complicated a phenomenon than it seems to be. There are three kinds of tears
1. Basal tears: It keeps our eyes constantly wet so that eyes are well lubricated. Basal tears is the paint that our eye-lids paints over our cornea while we blink our eyes
2. Reflex tears: It is produced when our eyes get irritated by any external things like dust. This is the same tears that we produce when we cut an onion (why?)
3. Emotional(Or psychic) tears: This is what we mention when we simply say 'tears' in literature. It is produced when we are hurt or emotionally affected.
This classification is based on the chemical composition of tears. The emotional tears contain the highest amount of hormones. Actually (emotional) tears is a defense mechanism of our body when it is unable to express through words. When we are affected by emotions, like loss of love, realization of a failure etc, our body produces a lot of chemicals and hormones that creates a feeling of being upset. On this, our nervous system stimulates the cranial nerve. This results in the production of neurotransmitters to the lacrimal glands which are situated on the upper outer portion of the cavity in brain where eyes are situated. Thus we cry. As long as the chemicals remain in our body, our state of being upset is continued. That is why the brain gives command to flow these chemicals out through tears. Once these chemicals are disposed, our body relieves from the upset state. Now you can guess, why we feel a bit calm after crying.
Can you guess why we end up crying after a heart-breaking laugh? It is obviously because, our lacrimal grands get squeezed by the intense muscular movement resulting from laugh.