It could be so many things; an eye infection, allergies, an upper respiratory infection (cold), conjunctivitis, etc. It would be very hard to figure the cause on your own. Since it's been happening for a while I'd say a trip to the vet is in order.
__________________
"If you have men who will exclude any of God’s creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men." — St. Francis of Assisi, 1181-1226