Aliens don't need water .....

Whenever I read articles about different planets/stars/galaxies and their suitability for extra-terresterial life, I am always surpised by why they always refer to the presence/absence of water or oxygen. They make it seem like a foregone conclusion that the lack of water and oxygen on the surface of Mars precludes the possibility of alien life on that planet. Why do we assume that aliens will have the same physiological needs as human beings ?

Maybe, aliens breathe Carbon Dioxide, maybe they eat rocks and thrive on temperatures like 4000F ... After all, there are organisms on earth that survive under those conditions. These aliens probably wont look very much like human beings, but then what is the probability that in our vast universe, two species of two-legged, upright mammals will look alike or have the same level of technological advance ?