Some people can't watch 3D (when I say can't, as in can't get the "benefit" of it) if they dont have binocular vision.
Binocular vision is where your eyes are working together, making a sort of panoramic shot of the world. If you dont have binocular vision, each eye sees separately and your brain sort of makes up the bit in the middle.
One of the reasons opthamologists try and "correct" young children with a squint is to achieve binocular vision by a certain age. If you dont have it by age 3 or 4 I think then you never achieve it, apparently.
The actual lazy eye thing can be fixed (generally) with surgery to tighten the muscle etc but a truly successful outcome gets the eyes working together.
If you dont have binocular vision, it doesn't really matter, but those 3D pictures which are loads of dots but with a picture "hidden" in them, mean nothing, parallel parking can be difficult, and you will probaby never be a ping-pong champion!
I dont have binocular vision, they never managed to achieve it, but my eyesight has improved so much since I was a little kid that I dont really care. To be able to live life with both eyes pointing in the "right" direction, and see perfectly with only the help of normal contact lenses in a fairly "mild" prescription is truly liberating compared to my original prognosis.