Learn more physics!
Why do air-resistance and friction resist motion?
Air resistance is really intuitive. Imagine driving a car surrounded by a bunch of other cars driving in random directions. Due to the relative velocities, you tend to bump into more cars that impede your motion than being bumped by other cars that speed up your motion. Same thing with air resistance, only on the molecular level.
Friction is a bit more tricky since there are several phenomena that are lumped under the word friction. Basically it's caused by the loss of kinetic energy due to inter-molecular interactions.
Again, there is a one-way trap door that tends to impede relative motion.
Check out the Wikipedia article on friction: http://en.wikipedia.org/wiki/Friction
It might seem surprising that all these different complicated effects always end up draining energy from big mechanical modes you can see and dumping it into subtle thermal modes- lots of small-scale hidden forms of energy. The idea behind why this happens is called the Second Law of Thermodynamics. We could explain it further if you're interested.
(published on 10/22/2007)
Follow-up on this answer.