Why is this generally thought?

[quantum mechanic]

Basically, what everyone else has said. Yes, it might be possible for life to exist without water, but a lot of important reactions to life *as we know it* take place in solution, and water is a very useful/common solvent (the "universal solvent," if you will). Additionally, it is in the middle of the pH scale and is very stable, so it can exist stably in the body for long periods of time without anything untoward happening. On top of all that, water forms hydrogen bonds all the time (it also breaks them all the time, but whatever), and hydrogen bonds are important to several critical-to-life-as-we-know-it molecules like DNA and RNA. Hydrogen and oxygen as elements are also important as they are components of many hydrocarbons and important agents in energy storage/release mechanisms like ATP/ADP transitions. Plus, of course, oxygen and hydrogen are both very common elements in the universe.

Bottom line: water is pretty versatile stuff. Life without water might exist, but it would probably be vastly different from us.

 

[AlkalineGamer]

I think they mean something along the lines of 'obvious (recognisable?) life'. There are theories about other forms of life, science fiction writers have to get their ideas from somewhere!

There's a pretty good programme narrated (also created?) by Professor Hawking that goes into alien life.

EDIT: The series is called 'Into the Universe'. Also deals with fun things like time-travel, speed of light, space exploration etc

Just watched the one on Aliens, thanks for mentioning it, it was very interesting.

Part of my inspiration for making this thread was one of the episodes of 'Wonders of the solar system' by brian cox, but it's a subject i've had in my head for a long time.

 

[Doclector]

I have thought this on multiple occasions. Sure, they have to rely on this to send probes to the most likely planets (just an example, of course) but maybe just once, humanity has to accept it's lack of understanding on the situation. That truly, we have no clue what kind of mysterious abyss we'll find out in space.

Or quite what we should try not to step on when we get there...

 

[Lenin211]

We only assume that life can only form in water because it is all that we have observed. It is logical then to only look for life on planets that have water. (that last sentence was a little off topic)