Simplest possibility: a cooling system leak. Check your radiator coolant level. Look for signs of leakage, tighten clamps to all hoses, top up with antifreeze. Otherwise, see a mechanic.
Several things may be causing this, it could be a combination of the following. Check to make sure you have a good 50/50 mix of coolant/water, the thermostat functions, the radiator fans/shroud are in good operating condition. There should also be no leaks anywhere and check your radiator cap.
One possible reason why a Jetta, or many other cars for that matter, might overheat could be due to a failure with the thermal switch for the cooling fan. At times, the plastic hose that connects to the engine could be the culprit.
Mainly, it lacks liquid coolant. The water pump is not working properly. The radiator or the water tubes are leaking. And, please do not forget to change the oil at a proper interval. Over used oil will definitely increase engine temperature. Basic checking and preventive maintenance service will certainly prolong your Jetta.
Either you have no coolant or your thermostat is inoperable. Depending on your location check your radiator fluid levels (there is a reservoir clearly labeled beneath the hood.