Interestingly, that one is nothing more than a quote from HPMOR. From chapter 63:

But "pessimistic" wasn't the correct word to describe Professor Quirrell's problem - if a problem it truly was, and not the superior wisdom of experience. But to Harry it looked like Professor Quirrell was constantly interpreting everything in the worst possible light. If you handed Professor Quirrell a glass that was 90% full, he'd tell you that the 10% empty part proved that no one really cared about water.