Too much sun, skin lotions that irritate, bad diet, deficit of sleep, or an excessive amount of sleep, not drinking enough water, your genes, or this is the aging process. That's quite common because just about everyone suffers with dry eyes as age increases. http://dntntranchinh.com/index.php/vi/index.php?language=vi&nv=statistics&nvvithemever=t&nv_redirect=aHR0cDovL2xvZy5Ua2ouanAvYW5hbHl6ZS5odG1sP2tleT10b3BfYXJhYmlhbiZ0bz1odHRwOi8vd3d3LnVyYmFuLU9mZnJvYWQubmV0L19fbWVkaWFfXy9qcy9uZXRzb2x0cmFkZW1hcmsucGhwP2Q9d3d3LnNwb3J0c2Jvb2suYWclMkZjdHIlMkZhY2N0bWd0JTJGcGwlMkZvcGVuTGluay5jdHIlM0ZjdHJQYWdlJTNEaHR0cHMlM0ElMkYlMkZ3d3cuVGhlcmFsaWZlLmNvbSUyRg