Have you noticed dark areas along your baseboards or under closet doors???
Be sure to use your vacuums's edge tool around your baseboards once a month to help prevent this condition, called "air infiltration".
Air infiltration is cased by air moving from place to place in your house while the heater or air conditioner is running. Your carpet is actually filtering the dirt out of the air.
So be sure to vacuum regularly and use that edge tool often. Not to mention having your carpet cleaned professionally every 6 months to a year. This will help improve not only the life of your carpet, but increase the air quality in your home.