Many. Our eyes are designed to see things in 3d. When you look at a computer/book etc for a long time (a 2d surface) you can strain your eyes or get dry eyes. Every 20 minutes rest your eyes for 20 seconds by looking at something at least 20 feet away. Get your eyes checked by a doctor too.
Various. Get evaluated by an eye care provider.
Focus alignment. Usually, eyestrain is caused by focus problems - either presbyopia or astigmatism. Less frequently, alignment of the eyes (strabismus) can cause strain. Best to be check by your ophthalmologist for focus and alignment, get the clearest glasses you can and rest when doing lengthy activities.