They can't focus together properly because they point in slightly different directions (possibly some derivative of a lazy eye - it was diagnosed when I was twelve, they told me they couldn't do anything about it until I was eighteen, and then they didn't). Both eyes work individually, but I can't make them work together. There would probably be an optimal distance where I can see 3D, but that's never going to be found in a cinema.

Because it's been an issue for so long, I've always tended to heavily favour one eye (usually the right) to the extent where I simply can't use both any more (my depth perception is legendarily awful); and since the entire point of 3D is the brain reinterpreting the slightly different images it gets from each eye... yeah. (The vision in both eyes is also deteriorating fast anyway, but that's a separate issue related to a severe bout of iritis about seven years ago. I really need to get to an optician...)

Also, the 3DS can fuck off.