What causes laptops to overheat?

By and large... the majority of laptops overheat because of this.

Image is from How to clean your laptop of dust and dirt .

The best analogy is understood if you have ever done laundry with modern appliances.... specifically a gas or electric clothes dryer.  Imagine what would happen if you ran and used that dryer every day, and never cleaned the lint trap.  How long before you KNOW you'd notice something was wrong... like the clothes never getting dry because the lint trap had never been emptied. 

No air flow.

This dust and debris comes from you, and all over.  That's lint from your clothes, and napkins, dust from the room, skin flakes, hair... and it is constantly getting sucked up into the fan, and blown against those really thin layers of metal there.  Like.. the lint getting caught in the screen of the lint trap.  And... the more dust the heat sink gathers, the smaller the space for air, so even MORE dust and debris gets caught instead of blowing through.  So... as this dust monster gets thicker, it gets progressively worse.

Then... there's the back pressure on the fan, of not being able to spin as fast as it should, because there's more resistance now, what with not being able to move the air.  Like a really bad air compressor, constantly drawing air and packing it in against the layer of debris.  That pressure takes a toll on the already relatively fragile laptop fan.

That's the main reason.  I'd say Poor Maintenance, but none of the manufacturers like talking about this.  Users just don't know.

Then there are those people who use laptops on their beds.  It doesn't matter if it a fanless Macbook that sheds heat through the casing.... the blanket or comforter does a great job at retaining heat (hence why we use it on our beds), and with the rest of us with laptops what have fans, the bedding and soft surface of the bed suffocates the laptop... blocking the fan intakes so there's no air to move through the heat sink.

That's the second-most popular cause of overheating.  This one... no.  I don't care how computer illiterate you might claim to be, a laptop overheating and shutting off on you every time you use it on the bed is almost inexcusable. 

You've never wondered why your laptop looks and feels like a hairdryer sometimes?  Seriously?  You've never picked up and looked at it from different angles?  That's a trick question.  Of course you haven't.  I know.  I'm a computer repairman.  I've heard you say that you never thought about where the air goes in or comes out... usually after I show you the layer of debris I peeled off your laptop heatsink and put it in a bag for you... to show you why some regular TLC goes a long way.