tow vehicle headlights keep shorting out

My tow vehicle headlights use the 9006 Halogen bulbs. I buy either Sylvania or GE bulbs. I don't touch the glass with my fingers when I replace the bulb (I heard that the oils from your skin can shorten the life of Halogen bulbs.) I see no signs of moisture in the headlamp housing. I have no other bulbs that require frequent replacement. My car doesn't have daytime running lights but I do drive with my headlights on day or night. I know that means more frequent replacements but I would think the bulbs should burn out at about the same rate. I've started buying the bulbs in two-packs & carry spares with me.

Does anyone have any ideas as to what's going on?