Heat

Your computer generates a lot of heat. Heat is the computers enemy. Dust and dirt try to hold the heat in, and that is bad. Fans try to get the heat away from anything that could be hurt by heat, but the fans often get plugged up with 'fluff'.

If you turn your computer on and off, or if it goes into power save mode, the system heats up with use, and then cools down when at rest. This heating and cooling cycle causes parts to expand and contract. This may cause some parts to 'walk' out of place, and some connections may become loose.

An example of what happens when a processor overheats can be seen in the article located at Tom's Hardware Guide. These devices contain millions of tiny transistors and are able to bake themselves into oblivion.