Well, first, you should know that when you get squinting eyes, it can cause wrinkles around the eyes and forehead. For the causes, excessive sunlight exposure can be a common one, so you can just check it with your own situation, for it can be dangerous to have squinting eyes. When having squinting eyes, many problems such as headache, the dry eyes, dark circles under your eyes, even the vision sight will be possible. At this moment, you should not go outside without sunglasses, and also some sunscreen should be used.
Well, as far as I know ,this is absolutely abnormal, because as you can see, your problem is officially called strabismus, which might make you squint your eyes when you focus your eyes on something. And generally speaking, that is derived inborn, or out of some bad habits, or serious myopia. Now, you should go to visit a doctor and maybe get a surgery to solve your problem.
I have squint eyes too! It really bothers me, because it looks very strange and annoying. Fortunately, it is controllable; with attention you can switch your eyes back to normal. Squint eyes means your eyes are not work coordinately, while one eye is focusing on one direction, the other may focusing on other directions. You said you always squint to see things, I think you have the high possibility of having squint eyes. I suggest you to make sure by having a examination in hospital.
If you have constant squint, you'd better treat it right away, a surgery may be needed. If you have intermittent squint, things are much better, you can treat it with special glasses or vision therapy.