There are two situations when we cry: when there is an irritant in our eye that needs to be washed away, and when we are under the influence of strong emotion.  Interestingly, the tears are chemically different for each situation.  When we cry from emotion our tears contain chemicals that are not present in our other tears.  These include manganese (a chemical depressant), leucine-enkephalin (an endorphin which helps control pain), and adrenocorticotrophic hormone (a hormone produced by bodies under stress).  When we release these toxins, we feel chemically and physically less depressed.  A good cry makes us feel better!