says my mind all the time, and it takes focus and memory for me to remember that isn't the one thing I wanted yesterday or the day before. "I only want one thing," always comes out of a context where there are other things present that I take for granted, or am currently disenchanted with.
As I think further, that statement "I only want one thing" really says "I want what I have now, and one more thing." It never occurs to my mind to list oxygen as a thing I want, but I'd die without it. But since I've never lacked it, my mind assumes it will always be there.