When you stare at the computer screen, you will forget to blink your eyes and the eye muscles are in tension for a long time which usually cause eye strain. You can shift your eyes to look at an object at least 20 feet away, for at least 20 seconds.
When you do too much work on computer, it will make your eyes tired. What's more, when you concentrate on the computer, you often blink less, which will make your eyes easy to dry out. So you often get eye strain when using computer.