Only man clogs his happiness with care, destroying what is with thoughts of what may be.

The specific harm named is anticipation. A perfectly good afternoon gets spent inside a future that has not happened, and by the time the future arrives you have already paid for it in advance, sometimes for a version that never occurs at all. No other creature manages to spoil a good day this efficiently.

Catching it is mostly a matter of noticing where your attention has gone. When you find yourself somewhere pleasant and not actually present, name what you are rehearsing and decide whether anything can be done about it today. If not, come back to the room deliberately, look at one thing in it, and let the future wait its turn.