Why is water necessary for alien life?

Every time I read an article about the possibility of life on other planets, the authors say that water is absolutely necessary to sustain life. Why would he think that other life forms would have ANY similarities to ours? Who's to say that they wouldn't need Hydrochloric Acid to stay alive? Who's to say that water wouldn't actually be poisonous to them? What if intelligent life is actually microscopic in size? Everyone seems to think that aliens would be about the same height as humans, that they would have eyes and a mouth and two arms and two legs. Why wouldn't they be floating of gas or huge rocks that process information in ways different from our five senses? Is there something about water that really makes it absolutely necessary for every possible type of life or is this just a reasonable guess for scientists to make?