Concealment preserves what is for Heaven

הרוצה שיתקיימו נכסיו יטע בהן אדר.

"One who wishes that his possessions endure should plant among them the adar tree" (Gittin 67a).

The Sefas Emes cites Chazal's counsel that property is preserved through the "adar" — a hint to the quality of concealment and inwardness rather than mere agriculture.

ע"פ הגמ' שנכסים נקראו ע"ש שנכסין מזה ונגלה לזה.

Based on the Gemara that possessions (nechasim) are so called because they are hidden (nichsin) from one and revealed to another.

The very word nechasim shares a root with concealment; wealth by its nature passes in and out of view, hidden from one person and revealed to the next.

וצריך האדם למסור כל חפציו להיות מתקיימין במדת ההצנע.

And a person must surrender all his concerns so that they endure through the trait of modest concealment (hatzne'a).

To make anything lasting, one must give it over to Hashem and keep it in the realm of the hidden — "walking modestly" (hatzne'a leches) rather than displaying it outwardly.

והעצה לזה להיות הרצון באמת להשי"ת בלבד.

And the counsel for this is that one's ratzon (will) be truly directed to Hashem alone.

The way to keep matters in the protected realm of concealment is to ensure one's inner will is purely for Hashem and not for outward recognition.

והכלל כי אם השי"ת מסייע להאדם שלא לגלות מעשיו.

And the principle is that if Hashem assists a person not to reveal his deeds.

When a person finds that his good actions remain hidden and undisplayed, this concealment is itself a form of divine assistance.

הוא סימן שהוא לש"ש באמת.

It is a sign that he is acting truly for the sake of Heaven (l'shem Shomayim).

That Hashem keeps one's deeds concealed is evidence that those deeds spring from a pure, inner motive for Heaven's sake alone.

וכמשמתערב רצונות אחרים אינו מתקיים במקום המכוסה:

And as soon as other desires become mixed in, it can no longer endure in the hidden place.

Once self-serving motives enter, the deed loses its rootedness in the concealed, enduring realm and forfeits its lasting protection.

Summary: True possessions and deeds endure only when kept in the realm of concealment, which depends on the inner will being purely for Hashem; Hashem's hiding of one's deeds is itself a sign of pure intent, while the intrusion of ulterior motives strips away that enduring, hidden protection.