Turning a light rapidly on, and off, or vibration, will shorten its life. The on and off doesn't have to be complete on and offs. If the power keeps changing (varies) several times a seconds, you won't see it; but, it will still have the same effect......shortened bulb life. There are several ways this can happen: the light relay may be unstable, causing unstable power. Wire connections, at connectors, to the lamp, to the grounding point, etc., will cause power interruptions.
The voltage needs to be carefully measured and observed, at the lamp connection, with the engine running (alternator running). AC and DC voltages should be checked. The grounding resistance should be carefully measured.
Switch the headlamp relays. If the other headlight burns out, next, the problem may be in that headlight relay.