What causes light bulbs to keep burning out in a porch light?

several things: It may have a poor connection at bulb or elsewhere; it may be rapid changes of temperature, or water or excessive moisture may cool bulb fast and blow it.& Don't leave out vandals.Also you may be using "bulbs from china or similar country."you can buy bulbs rated at a higher voltage like 130vac which are heavier elements and usually last dbl hours.