The flywheel is the part of the clutch that connects to the engine and helps lock the engine to the transmission input shaft so they spin at the same speed. No clutch will last forever, and the flywheel is susceptible from wear and warping due to heat caused from friction. When the engine is under pressure from driving up a hill, towing something, or accelerating to pass, the clutch is more at risk of slipping. So, it gets hotter. And the hotter it gets, the more stress this puts on the flywheel.