Myopia is also called nearsightedness, it is a refractive disorder where distant object cannot be seen clearly. This condition occurs because light rays coming to an eye from distant object focus in front of retina and the resulting image on the retina is
blurry. The most common reason for myopia is elongation of an eyeball and normal refractive power becomes too strong for an eye. The images formed on the retina are blurry and therefore cannot be seen clearly. This form of myopia is known as axial myopia. In some rare cases it is possible that refractive power of an eye is too strong for a normal size of an eyeball and images from distant objects again focus in front of retina instead on it. The reason is either a strong refractive power of cornea (for example in condition known as keratoconus), or lens (for example condition known as lenticonus. Source:http://www.refractiveeyedisorders.com/myopia-nearsightedness/
Many people think that people get myopia because they do too much close work, such as, reading or sitting too close to the TV or the computer.However, nearsightedness may also be caused by eye diseases, such as, cataracts and keratoconus.
Some myopia are cause by genetic factor,but most of the myopia are caused by daily habit of using eyes such as reading too much in the dim light or sitting too close to the computer.The ciliaris of your eyes are strained in a long time and can't get enough rest.Then it lose the elasticity of adjustment to see far away.
If both of your parents are nearsighted,then chance increases that you will also develop myopia.The subsequent environment can also cause the myopia to happen like too much close work. When you are trying to see things far away,they look blurry,then you are suffering myopia.