[andrewas]

It causes motion sickness because theres a disparity between the motion you see and the motion you feel.

This is not quite the same as travel sickness. Most people get travel sick to some degree, but far fewer get sick playing first person. Theres a strong psychological component to it as well, otherwise the same effect would be observed in third person.

Some sufferers report that changing monitors, sitting further away or sometimes just taking a break every half an hour helps. You can probably find more info on google.