Why does my compressor turn on and off?

On a lot of cars the cycling of the compressor "on and off" is normal as long as it is providing enough cold air to the passenger compartment. However, rapid cycling is sometimes the sign of a refrigerant charge that is just a little bit low. If you are still concerned, have a competent shop hook up their gauges and do a quick pressure test. They might even do it free if they are nice and honest like me.