If your hair lacks shine, it means it has become dull. Hair becomes dull due to a variety of factors. Generally speaking, hair dulls due to a lack of moisture: dry hair reflects no light and therefore lacks shine. The various causes of dry hair also lead to a lack of shine.
So it's important to moisturize your hair both inside and out. An unbalanced diet, overexposure to the sun, stress or even excessive use of aggressive products can all affect the shine of your hair. That's why it's so important to maintain your hair's shine with the right routine.