You probably have cloth seats and when you slide in and out you build a static charge. When you get out and touch the metal of your car, you complete the circuit and the electricity built up in you is released into the car. h:

It could also be that your tires build static electricity from rolling down the road. When you touch the car, you are grounded and that completes the circuit.

Nothing to worry about... it's normal. Just be sure to touch somewhere on your car before you go filling up for gas. :shocked: