What causes styes?

Retained Secretions: Along the upper and lower eyelids are dozens of meibomian glands. There job is to secrete an oily substance to mix with your natural tears. In the case of a stye, aka chalazion, the secretions become so thick (think of dried toothpaste) the gland can't empty its secretions. The gland gets bigger due to built up secretions causing redness, pain and inflammation.

Clogged gland: A sty is the result of a clogged mebomian gland of the eyelid. This gland makes lipid and oil to coat the surface of the eye to keep it moist, protect from the environment, and the oil prevents rapid evaporation. When the mebomian gland clogs the oil backs up forming a swollen gland that is a red swollen nodule in the eyelid. Cause is usually genetic and inflammation.