Even though your description is pretty general, at least it doesn't sound as serious as if the discharge was purulent (yellow-green, like pus). A lot depends on the breed of your dog, its age, and whether you keep the dog inside or outside.
Some of the short-faced dogs tend to have a lot more eye discharge, even when nothing is wrong. Other breeds with long hairs around the eyes will have more watery/mucus discharge just from the irritation of the hair. Outside dogs experience more wind-blown dust, pollen, etc., and those can cause watery discharge.
As long as your dog isn't squinting one or both of its eyes, the discharge isn't purulent, and there isn't any redness around the eyes, this is most likely the result of a mild allergic response or a physical irritant that causing increased tear production.
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