I agree that it's probably allergies BUT drainage from a pet's eyes could also result from infections elsewhere, namely teeth or ears. Also there are several problems that would mean the dog is uncomfortable - like possibly his eyelashes are rubbing against his eyes, his eyes (even though they're producing mucus) could very well be too dry which can cause permanent vision impairment, conjunctivitis, lots of things and the dog has no way to help himself, he is totally dependent on his owner to find out what's going on and help him get relief.

I consider myself conservative when it comes to treating pet health concerns but it seldom plays out that waiting ends-up saving money with the veterinarian. Usually if you can nip something in the bud it'll actually be more economical and the pet will be spared more discomfort.