Why is liquid water considered a requirement for life?
The reason for which water is needed for Life as we know it (and we have not been able to successfuly imagine a different model from that of DNA that has ability to self-replicate, react to the ambient, and evolve), is that water is the "universal solvent".

That means that most chemical reactions needed for life happen on a liquid environment, more specifically a water-based environment for most of them.