why does my left headlight always burn out?

The number one cause of repeat failures is improper handling of the new bulb. You must not touch the glass part. The oil from your fingerprints will burn through the glass and cause it to shatter. If you do touch it, it can be cleaned with brake parts cleaner and a rag.

A second, less common cause is water build up inside the housing. Some manufacturers have come out with a service bulletin at one time or another about drilling a small drain hole in the bottom of the housing to let condensation drain out.