Brakes will wear down naturally with long term use. Sometimes they can wear down abnormally quickly for several reasons. If the rotors are worn, burnt, or have any other damage they can create rapid pad wear and need to be replaced. Cheap pads generally do not wear very long and sometimes can contain chunks of metal that eat into the rotors. Excessive hard braking, or driving with two feet and resting your left foot on the brake, can also contribute to faster brake wear. If you detect anything unusual with your brakes, don't hesitate to take your vehicle in to get the brakes looked at. This will ensure your safety on the road.