It could be any one of a number of things. For example, it could be a primary infection caused by a bacteria that she can't shake off on her own for some reason. It could be an allergic response to something outside or in. Outside it could be any one or more than one of the same things people are allergic to, such as the pollens from different plants or trees. Inside, it might be a dye in a plastic food or water bowl or an irritant in the bedding she sleeps on. If she rides with her head out the car window, this could also do it. Or if she has very prominant eyes (like those of pugs, and many small breeds) and if you notice a lot of wetness/tearing, it might be that instead of the tears keeping her eyes clean and lubricated, they're just spilling over the lower lid instead because the size of her eyes is interfering with this system.
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