Mine was caused by doing stressful grad school things, keeping ridiculously bad sleep schedules, and staring at a too-bright monitor for dozens of hours per day

Mine was caused by doing stressful grad school things, keeping ridiculously bad sleep schedules, and staring at a too-bright monitor for dozens of hours per day. These added up to rubbing my eyes a lot, which led directly to styes. After reducing my monitor brightness, the problem went away.