Causes...
We know what the effects of eyestrain are and we don't want to feel those. So why do we get headaches if we look at a computer too long, or why do our eyes turn red after editing a long term paper?
Here's Why
The monitor that you are looking at this web page with is set up with pixels... tiny little dots. As you can tell with this large "A" underneath, you see the ridges. Those ridges are pixels. You eyes don't like pixels. They like smooth edges. Even though your font maybe small and the ridges are not noticeable, your eyes are constantly trying to make them smooth. So stare for about 15 seconds at the "A"...if you stare at the large "A" underneath, you eyes will begin to strain and become uncomfortable.
Now, imagine that your eyes are constantly trying to do that. Maybe not to the scale that we have just experienced, but the toll on your eyes will eventually show.
Another Reason
According to an article in Scientific American, your eyes do not blink as often as they should when looking at a computer screen. So ...BLINK!!! What happens again is that our eyes become so entranced with trying to make that ridge clearer. Scientific American said, "The normal blink rate is around 20 times per minute but using a computer can drop it to as low as seven..." So once again, BLINK!!!