Eye allergies, just like all allergies, occur when your immune system overreacts to allergens that you come into contact with. In the case of your eyes, allergens like grass, weeds, trees, pet dander, dust mites and mold can all go airborne and hit your peepers.
Once they come into contact with your eyes, they react defensively to the invasion and mast cells release a chemical called histamine. Once this happens, the blood vessels swell up and there goes the puffy, itchy, watery eyes. Thanks a lot, over-reactive immune system!