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.