The causes of styes may include :
Uncorrected spectacle number.
Use of wrong number spectacles.
Eyestrain due to prolonged near work or eye muscle weakness or imbalance.
Blepharitis which is a chronic infection with dandruff- like flecks over the eyelashes or lid edges.
Exposure to dust or pollution.
Diabetes.
Hormonal changes especially around puberty, in women or on treatment with hormonal medicines.
Skin conditions like acne rosacea
Sometimes there may be no obvious cause.