Another aspect of the question is "why we cry" as responses to emotional or physical
stimuli that have nothing to do with eye injury or
contamination.
Drawing from
cultural anthropology and
evolutionary psychology one might conclude that crying is a method of communication that had to exist for
preverbal cultures and modern human beings when in a preverbal phase of development.
If a member of a
tribe was suffering in some way that could be addressed by another member of the
clan, prior to the development of
language, it would be to the overall
evolutionary advantage of this
genetic branch to have a mechanism by which this could be
communicated.
Similarly, a baby who cries communicates (quite effectively, to the point of being a kind of
low-tech auditory spam) his
biological needs of feeding, changing, and the like.
Crying is also useful as a kind of
intrinsic language that is generally difficult to
fake. It is generally difficult for someone to ignore another person who is crying, probably due both to an evolutionary
trigger of
empathy, and also a more
cognitive understanding that the person is likely not making a false or
dissembling presentation of how he/she feels.
An interesting unanswered question is why evolution might have selected this
specific physiological response, rather than another, such as, say, twitching elbows to indicate
distress or strong
emotion. Perhaps we are biologically geared toward looking at others' faces to gauge their "state", as a general
heuristic.