It might be that your caliper piston is not fully releasing when you let off the brake pedal. If it's the piston, then basically it's as if you are 'riding' your brakes everytime you take your car out. The excessive heat will glaze your pads/rotors essentially rendering that side to the point that it's almost useless, in which case your passenger side will start to compensate and wear more quickly. How many miles do you have on your car? Maybe you should consider rebuilding both front calipers. Otherwise, you'll risk damaging those new rotors.