A light bulb only lasts as long as it's used...
|Meaning, if you are in cold temperature, and you drive several times a day - your bulb gets hot and cold several times each day, changing it's size over and over until it doesnt feel like doing this anymore and it just breaks
If you live in the heat and your lights are on forever, then you might blow your bulb quicker than others might
If you touch your light bulbs glass while putting it in, the oils on your fingers can shorten the life of the bulb to only a few seconds or for a much shorter lifespan than if you had NOT touched the bulbs
If you have someone pounding on your car to remove your suspension or do major bodywork, this may cause the filament in your lights to break (this is a common reason why neon are no longer working after major bodywork has been done)
If you get rear-ended (or any other sort of accident that may jerk your bulbs during operation), your bulbs also might get damaged since they are most fragile when hot.
If you are driving over poor quality roads all the time, your headlight have several more opportunities to go bad, quicker than someone's who drives over smooth roads.
If you start your car or turn off your car while your lights are on, the surge of electricity can damage the bulbs too.
If you have an electrical short on the same circuit that your lighting is installed too, and the fuse does not break, as it should, this may cause your light bulb (or light bulbs) to also need replacement.
There are many other causes for why headlights go bad... but these are only what I can think of now.
I currently have PIA in my fogs and drive with them ON at all times, DAY or NIGHT... theyve lasted since December 2000 and 46,000 of Los Angeles stop-go traffic and 100d temperatures.
I had Showoff in my head lights - never any problems for 2 years...
I then upgraded to MTEC in my HI and LOWs and still no problems after eight months and going
I've heard stories from ALL bulb brands about them going bad at one time or another... so its just basically what brand you want to deal with.
PIA bulbs are usually sold in singles and the price range is about $19 to $75 per EACH single bulb. I paid almost $60 just for my set (two) of PAI H3 FOGs ONLY.
If you cannot afford the costly price of PIA, I highly recommend MTEC all-the-way-around