Every time you slide in to your seat you will acquire static electricity, and thus your body will become charged with e-. Now when the air outside is humid, the water in the air quickly neutralizes your body's charge when you are in contact with the outside air. When the air is dry (usually during winter), there is no water, so your body stays charged, so when you touch a metal object, the electrons from your body will jump to the metal and thus shock you. This is completely normal. Now, to avoid getting shocked, do this. Before stepping out of the car, open the door and grab the roof with your hand, then proceed to exit with your hand on the roof. When you are completely out of the car, you can remove your hand and touch the car again and it wont shock you, because all electrons have already transferred to the metal.
One thing to keep in mind. Whenever you are at the gas station, make sure you are neutralized before you touch the pump, otherwise the spark can create fire.