It cannot be death, for a fear of death is a fear of being alive. It cannot be pain, for that's to fear feeling anything at all. Nor can it be loneliness, poverty, illness. All of these invite denial. Denial of one's condition, denial of uncertainty, denial of impermanence.
The only fear is of a life unlived. It's up to each of us to interpret what that means for ourselves, just as we must each bear the burden of being honest with ourselves.
So when confronted with illusions or distractions, numbing comforts of flesh or entrancing fancies of mind, may I recall this one fear.
Yet when seeking the boundary between play and frivolity, allow me to err on the side of levity. Allow me the mistakes of perpetual youth just as I am afforded the chance to grow no matter how old I become or how much I think I know.