There is a good summary of user testing. It is from the book called Influence by Robert B. Cialdini, PhD, and it points out something self-evident yet profound. It says that people determine what is correct by checking out what other people think is correct. It sounded wrong the first time I read it because I always thought that the right way to check if you are correct is to use scientific methods. Then I read it a few more times until it occurred that while it is not science grade, it is very true in everyday life, and I think it is true with user testing.
When we do qualitative user testing, we are essentially interested in what people think and how they react to our designs. So, we are ultimately testing if our answer to users' problems matches theirs. Having this thought in my mind helps me to put things in a simpler perspective. We may think that we have found THE SOLUTION, but until we see that they actually find it useful, we cannot know for sure.