Does constantly squinting have negative effects on your eyes?

No. Generally, the squinting of one's eye is not bad. It is mostly for one of two reasons: the need for a refractive correction (glasses or contacts) or light sensitivity in sunlight or bright lights. If when you squint you can see better, you should see an eye doctor for refractive correction. If you see just as well with/without squinting obtain sunglasses, polarized with 100% uva & uvb blocker.