There is a tenet in ancient yoga that states that if a body part is placed in a certain position for a long period of time, that it will get used to being that way, and it will always remain that way.
In light of the above, consider that when you read books, or are at the computer for long periods of time, your eye muscles contract in order to focus at a near distance. It works like this: to focus on nearby objects, the eye muscles must contract, lengthening the eyeball to see better at a close distance. To focus more on distant objects, the eye muscles must relax, and the eyeball shortens in order to see better at a far distance.
If you do these activities (reading books, working in front of a computer, etc) for long periods of time, and very regularly, then ever your eyes will get used to focusing only at near distances, the eye muscles will get used to being in a state of contraction, and your far vision will start to deteriorate and become blurry.
Understand that evolution designed your eyes to focus and both near and far distances. Primitive man had to hunt and be able to see clearly at far distances, but modern man spends a lot of time indoors with artificial lighting and focusing predominately at near distances. That is why there is now widespread myopia and more and more people who need glasses. Because people do not use their eyes the way evolution designed them to.
The good news is that nearsightedness can be reversed. What you need to do is start using your eyes, without your glasses, and focus more at far distances. Going for a walk outdoors and looking at distant scenery is a good way to improve your eyes ability to see at a distance. There are also specific eye exercises you can do that loosen the tension and relax your eye muscles so that you can regain your ability to see at far distances.
Understand that everyone is meant to be able to see clearly at all distances, this is what is normal and natural. Wearing glasses is not a natural state. Moreover, wearing glasses perpetuates the problem because it makes your eyes get used to seeing imperfectly.
There are many principals of natural eyesight improvement. If you understand and apply these principles then there is no reason why you too can not improve your eyesight.