Why does the dryer overheat?

First guess is not enough airflow -- See how much air is blowing out the exhaust vent. If it seems weak, first make sure there is no excess lint in the exhaust path (check inside the dryer as well as the exhaust hose). If there still isn't enough air moving, the blower fan is likely loose on its shaft. If the dryer is overheating and you have very strong airflow, then the temperature detectors in the exhaust path are probably not ever telling the heating element it's warm enough. Check for loose wires, or you might have to replace the detector.