Could be allergies or an infection. Best to see your local vet or bring the animal in to the store as an employee may be able to recommend a product.
Most common cause is an environmental irritant - dust, fumes, pollen, or even just dry air. Regular lubricating eye drops are often helpful. If you start seeing distinct redness, swelling, or a greenish discharge, an infection is likely - in that case see your veterinarian for medicated drops.
Could just be allergies like a person. Look up the dosage for your dogs weight and benadryl.
Thanks! Your answer is awaiting moderation.