As the other guy said it is not a rule but a guideline. If you are spending more than 30% of your total income in housing costs your expenses are out of balance and other things suffer such as your savings.

Heck some people then look at it as if it was a goal, which is also off the mark. I make good money, I could afford a lot more house but it is awesome that my current mortgage payment is only 12% of my gross, it makes it super easy to find money to save.