Our brain processes 3D vision through a complex visual pathway, and clear vision is just one step on that process.  We have to have 2 eyes to see in 3D because the information that both eyes receive must be added together.  Try opening and closing one eye and then the other.  You will see that it looks like the world moves as you switch between your eyes.  These 2 different images are combined together in the brain to create a three dimensional image.  This means that where your eyes are focusing must be corresponding (if one eye has an entirely different view than the other, there is no similar image for the brain to fuse between the eyes!). 

Each image from each eye is sent through a visual pathway that journeys from eye to the occipital lobe  in the back of the brain.  Once it reaches the brain receptors, the brain must interpret and fuse the information, and then signal to a different lobe of the brain what kind of motor adjustments need to be made to respond to the 3D surrounding.  It's an intense process with many possible interruptions along the way.  

Reasons you may NOT have 3D vision

Not everyone can see in depth, either with 3D movie glasses or even with their daily vision.  The most common causes of not having depth perception (or stereopsis) are:

1) Blurry Vision:  Refractive errors like myopia, astigmatism, and hyperopia can cause a blurry image to  he brain which inhibits depth perception.  This is especially true when the image from one eye is more blurry than the image from the other.  If you notice a big visual difference when comparing your two eyes, it is definitely necessary to see your eye doctor.  Even if you can see 20/20 with both eyes together, you can never have full stereopsis with 1 eye processing a blurry image.

2) Lazy Eye (Amblyopia): Amblyopia is a medical condition where one eye sees at least 1 line worse than the better seeing eye, despite best glasses correction.  Sometimes when a person has a high prescription on one eye, or an eye turn, that eye does not develop normal visual connections to the brain.  The result is a "lazy eye" that can't see 20/20 vision even with glasses or contact lenses.  The best way to treat amblyopia is to catch it early and try to clear the image to the weaker eye.  With vision therapy treatments, it may be possible to develop the amblyopic eye back to 20/20 vision, though the process is typically long and involved.

3) Eye Turn (Strabismus):  An eye turn occurs when one eye focuses at a different point in space than the fellow eye.  Typically this results from a misalignment of the eye muscles, but it can also result from a high prescription or a high amount of eye strain.  Strabismus can also be treated with vision therapy.  In some cases, surgery can be performed to correct the misaligned eye, but even after surgery a patient may be unable to achieve depth perception without vision therapy.

4) Binocular Vision Issues:  I'm going to lump all of these categories together because when your eyes aren't working perfectly together, you won't have true binocular vision, and you won't have depth perception.  People with trouble focusing at near (accommodative issues) and those whose eyes tend to  lose the ability to focus equally with fatigue (convergence issues) can both develop issues with 3D vision as their eyes fatigue.  Do your eyes get tired when you read or use the computer?  Do you see double after reading or near activities?  Do you get a headache when using your eyes for prolonged near work?  If you answered yes to any of these issues, you need to visit your eye doctor for a full binocular vision work up.