I really hate it when math book authors say that something is trivial or obvious. If it wasn't obvious to you, then it only serves to make you feel stupid. If it was obvious, then it only serves to make you feel superior for a brief moment (compared to a stupider imaginary person, I guess). (And if it was truly obvious, then why say it at all?)

I think this is rooted in a deep insecurity to always have an answer, to always appear like you're on your game. (Why did evolution give us this weakness? Acting this way must have been useful for some survival reason.)