Why would a motherboard need to be replaced?

[zaplzaplza]

I just found out that my computer's motherboard needs to be replaced. What causes the motherboard to go bad? This is the second computer that I have had that needed a new motherboard- the current one a 1 1/2 year old sony vaio laptop and a 5 year old gateway desktop. How can I prevent the motherboard from going out on my two other computers?

[Raddamant]

Build quality and temperature. Not all mobos are created equal. Some are simply built more robustly than others.

High temps wear out electronic components quickly. Keep the inside of your computers clear of dust and make sure there is plenty of space around your machines to keep air flowing very freely.
Turning your machines on and off frequently is also hard on components. Circuits heat up when running and actually expand a little. When turned off, they cool and then contract again. It's like taking a copper wire and bending it back and forth- at first it just bends, but eventually it will break.