There is no scientific evidence to show that all people need this amount of water. The simple fact is that people need to drink exactly as much as necessary to quench their thirst. Thirst is there for that reason. Do you think you know better than your own body?

I actually bothered to search the scientific literature about this.
There is an interesting article in http://ajpregu.physiology.org/cgi/content/full/283/5/R993.

Doctors seem to agree, there is no real need to drink excessive amounts of water.