You remember the movie "City Slickers" from a while back? Jack Palance's character mentions "that one thing," the most important thing. Once you know what that one thing is, that's what you devote your life to pursuing, the most important thing in life.
Giving that some thought, I came to some conclusions. Say, for example: Tragedy is not the most important thing.
I'd think that The Most Important Thing would have to be scarce. Tragedy is not scarce. Everyone has some tragedy in their life. Often it's something quite large and disturbing and saddening. But it isn't That One Thing.
Wait, am I saying that Tragedy isn't important? Well, no. Tragedy is Tragic, that's why they call it that. But That One Thing would have to be something scarce. Everybody's got some Tragedy, so it ain't scarce. Even the Bible contends, when there were (arguably) only four people in the entire world, there was already a murder, a tragedy. (And then there were three.) Not scarce; and in some ways, things aren't much better.
How about love? Well, Love is Lovely, that's why they call it that. Scarce? Depends on how you measure scarcity. You could argue there isn't quite enough, but I don't know if you can argue it's Scarce.
So, what is That One Thing? (Drumroll, please!)
I have no idea.