Finally, something I know about. I am a thermal engineer for a technology company. I build and install some of the world's largest supercomputers.

People here are talking about why computers generate heat, but not why they overheat.

There are four major reasons computers overheat:

• Time and heat cycles. Between the processor and the heat sink (the part charged with dissipating the heat), the manufacturer places a grease that helps to transfer heat. It fills the microscopic imperfections on the processor cap and heat sink. As the computer heats up and cools down, this grease gets "pumped" out from the center. The center is where the most heat is generated. Even if everything else is perfect, after enough heat cycles the processor will start overheating.
• Corrosion or damage to heat pipes/vapor chambers. Most computers, and nearly all laptops, use a small vapor/liquid system to help spread or transfer the heat somewhere else. Sometimes, these heat pipes or vapor chambers corrode or otherwise leak, making them worthless. At that point, the computer quickly quits dissipating heat properly.
• Fan failure. Your fan can simply stop operating. They are made by the lowest bidder and are not particularly reliable. If they fail and the warranty is used, a fan is cheap. If they fail and you buy a new computer, the company might make a new sale.
• Improper maintenance. Over time, the heat sink, fan, air intake, and air exhaust act as filters. All the hair, dust bunnies, lint, and other particles that you see around your house also end up in the air system. As this clogs up, the fan cannot pull enough air. A little shot of air every now and again will help.