Myopia develops when the eye exhibits unusual structural issues. In some children, the problem might arise if the cornea, which helps incoming light focus at a single point, is overly curved. Other children might have longer than average eyes. A naturally healthy eye assumes a nearly perfectly round shape, but a longer eye makes it more difficult for light to converge at the retina as needed for clear vision.
The exact cause of nearsightedness is unknown, but two factors may be primarily responsible for its development, heredity and visual stress. There is significant evidence that many people inherit nearsightedness, or at least the tendency to develop nearsightedness. If one or both parents are nearsighted, there is an increased chance their children will be nearsighted.Even though the tendency to develop nearsightedness may be inherited, its actual development may be affected by how a person uses his or her eyes. Individuals who spend considerable time reading, working at a computer, or doing other intense close visual work may be more likely to develop nearsightedness.
Myopia occurs when the eyeball is too long, relative to the focusing power of the cornea and lens of the eye. This causes light rays to focus at a point in front of the retina, rather than directly on its surface.Nearsightedness also can be caused by the cornea and/or lens being too curved for the length of the eyeball. In some cases, myopia is due to a combination of these factors.Myopia typically begins in childhood and you may have a higher risk if your parents are nearsighted. In most cases, nearsightedness stabilizes in early adulthood but sometimes it continues to progress with age.
Generally speaking, the cause for myopia may be the genes. That is to say if your parents are both short-sightedness, you are own a higher risk to have myopia. Moreover, we always ignore the fact that our habits and our environment also play a role. Our visual habits and our environment play a pretty role in all of this. When we are looking up close, we are creating something called accommodative strain in the eyes. When we do this for excessively long periods of time it triggers the eyes to change in order to reduce the amount of accommodative strain. This change is to become nearsighted, usually by increasing the axial length of the eyes. That's my take on it. Hope it help