While you are right in saying that our assumptions about where and how life can or will evolve are probably to limited, there is some logic in saying that life will probably require water, oxygen, ect. ect. We are learning now (in the last decade or so) that life is a lot more tough than we thought. We've found life thriving around volcanic vents deep below the ocean where no sunlight ever shines (Europa anyone?), we've found it in Antarctica, in ice, frozen in rocks, deep underground in pools of icy water (Mars anyone?), we've also found it in scalding hot lakes, volcanos and other hot and seemingly unhabitable places (Venus, Io, anyone?). Life very well may exist on such places as Europa or Mars, less likely but still possible places are the Jupiter, Titan, and possibly others. However, here is the reason why we assume that it probably needs the same elements it does on earth: we, the humans, along with every other plant and animal on this planet are made of some of the most abundent elements in the universe. Carbon, oxygen, nitrogen, hydrogen, helium, methane and so forth. Were we made of such things as uranium or other rare elements, then it would be safe to assume that other life would be made of other stuff. Now granted, most of the matter in the universe is hydrogen, but in a solar system most of this element goes into the star(s) of that system, and a great amount of what is left goes into the gas giants. The terrestial planets (rocky) are made of the most common elements after helium and hydrogen, those previously mentioned, which are the same things we are made out of (remember in Star Trek 1? "Carbon based life forms.") An of course, one of the main ingredients for life here on planet Earth is the simplest compound of two of the most common elements, hydrogen and oxygen make water. And we know water is an abundent compound throughout the solar system, comets, Mars, Europa, Titan, Trition, Jupiter, Saturn, ect. ect. So basicly, we assume that those elements are needed because we are made of them and they are probably abundent in most solar systems, so therefore it is reasonable to assume that other forms of life has just as good of a shot of coming into existence as we do (given the right conditions, but this is under heavy debate so we won't go there). Hope this helps you out some.

-Paul

: If you have ever read a book that mentioned extraterrestrial life,you'd probaly notice that they all say for life to exist on other worlds,there has to be Water,Nitrogen,Oxygen,and warm temparature.Why??Just because that's what life on Earth needs,that doesn't mean that it's a universal need.If life will evolve on a planet,it will evolve to fit into its enviroment.