Why do I have a constant squint in my eyes?

Few possibilities. If you find yourself squinting to see things better, you likely have some kind of vision problem, and a trip to your ophthalmologist will determine if it can be corrected with glasses, or if it represents something more serious. If you mean that your eye is squinting involuntarily due to bright light, or that your eyes look different from each other, your doctor should be able to diagnose that too.