Answers:
cataract
Allergies and infections
Infection, irritation, want of irrigation and milk.
Having some blocked scratch ducts(I know, that sounds odd) will rationale the eye to become a "dry eye" rubbing against the eyelid. The other ducts will over-produce extra mucous to lubricate the eyeball to protect it.
Certain drugs will do extra mucous
Also contaminants above ground that stop on the eye(the gash ducts will try to clean this crap out)