It seems as if your dog is having a genetic defect known as Entropion. In this situation eyelids turn inward and thus cause runny eyes when lashes brush against eye surface frequently. Few dogs naturally grow hair in their eyes or some times eyelashes grow in such angle, which irritate dog's eye continuously. Irritation is not always there. There can be an allergy problem as well but as you did not mention any other symptom so it looks normal. Do not worry. If you note any other symptom then take your dog to vet at once.
