I'm not sure what the formal, official medical reason is, but smoking of any kind is drying to your body, and I know weed smoking is definitely that way because it's often recommended medically to help dry up secretions and saliva. I have dry eyes, too, and it's miserable. My eye doctor put in some tear duct plugs to try and improve the situation, and they were more miserable than the dry eye problem itself. I took out the plugs with some tweezers and now just use lubricant eye drops every chance I get. I'm not a current weed smoker, so that doesn't trouble me, but if I were, I know it'd make the problem worse.

The Erowid Web site (link below) said the drying effects of cannabis can vary from strain to strain so you might try a different strain and see if that makes a difference. Also, adding more essential fatty acids to your diet can help the dry eye problem, too. Those include Omega 3- and Omega 6-rich foods like salmon, tuna, nuts, and flax seed oil. Might be worth a try.
http://www.erowid.org/plants/cannabi..._effects.shtml