Why do scientists think life requires water? As far as I know, scientists seeking extraterrestial life are simply looking for plantets containing water in liquid state. To be honest, I find this weird.

It is true that life - in the form we know on Earth - cannot exists without water. But how can one proof that such life isn't but a special case out of a bunch of other different types of life?

Actually, I think we can come out with at least one example of a "organism-like thingy" that would not require liquid water. Well, we could build a simple robot, whose "natural ecosystem" would be a junkyard (for mechanical, electric and electronic garbage). That robot would would fuel itself with solar energy, and reproduce by seeking out appropiate parts in the junkyard and building a copy of itself from them. Of course, that's just a theoretical example, I know it has many faoults, but I still hope the idea is clear - it should be indeed possible for life to exists without water.

I have also heard (sorry, I can't remember the source) that theoretically, there could be an organism whose metabolism would rely on sulphur.

So, unless scientists can proove that no life can exists without water (any life; not just Earth-like life), why do they keep looking just for water?

Unless, of course, we define "life" as "any organism functioning like Earth organisms; therefore, metabolising organic structures with enzymes, built of protein, requiring water, etc" 

BTW. This question is a little bit different than Why is liquid water considered a requirement for life? . The answers there were pointing out that water is a good solvent. And I'm (also) asking why is any similar solvent considered needed. See my example above.
When you lose your keys on a dark night you look where the probability of finding them is highest (which is a combination of being likely to see them with their probability of being there), which is not necessarily where they are most likely to be. In the case of the search for extra-terrestrial life we have a fairly good idea what the signature of carbon based life using water as its primary solvent (conventional life) looks like. We also know that such life is possible. Other unconventional forms of life could exist but we have a very poor idea of what signature to look for, so we devote most of our effort to looking for the signature we will recognise.

We do not need to prove that unconventional life is impossible in order to spend most effort on searching for the conventional. It is sufficient to suspect that the probability of finding such life is higher than unconventional just because the probability of detection is so much higher. If we find nothing looking for conventional life, then a larger proportion of the effort will be devoted to searching for the unconventional.