Torah lishmah engraves the heart

איתא בשם הרה"ק ר' בער ז"ל פי' הפסוק אשרי אדם עוז לו בך מסילות בלבבם.

It is brought in the name of the holy Rebbe R' Ber, of blessed memory, an explanation of the pasuk: "Fortunate is the man whose strength is in You, in whose heart are the highways" (Tehillim 84:6).

The Sefas Emes opens by citing the holy R' Ber on the verse that praises the person who carries within his heart "highways" — paved paths leading upward to Hashem.

שמי לו מסילות בלבבו.

That is, one who has highways within his heart.

The "highways in the heart" describe a person whose inner self is already cleared and prepared as a roadway toward Hashem.

אז התורה שלו יש לו דביקות בהבורא ית' וזה עוז לו בך.

Then his Torah is truly his own, and he has deveikus (cleaving) to the Creator, blessed be He — and this is "whose strength is in You."

When a person has prepared these inner pathways, his Torah becomes internalized and his very strength flows from his attachment to Hashem.

ע"ש בס' תהלות ישראל בשמו.

See there, in the sefer Tehilos Yisrael, in his name.

The Sefas Emes directs us to the source where R' Ber's explanation is recorded.

וכמו כן יש לדרוש מרישא לסיפא שמי שלומד לשמה ויש לו דביקות בו ית' בתורה.

And likewise one may expound it from the end back to the beginning: that one who learns Torah lishmah (for its own sake) and has deveikus to Him, blessed be He, through the Torah —

The Sefas Emes now reverses the order of the verse, reading the cause and effect the other way around: deveikus achieved through learning lishmah.

אז התורה מניח בו רשימה וחקיקה בלב.

Then the Torah leaves within him an impression and an engraving upon the heart.

Learning lishmah engraves the Torah permanently into the heart — this engraving is precisely the "highways" of the verse, now formed from within.

כמ"ש במעלות העוסק בתורה לשמה זוכה לדברים הרבה ומזדכך לבו לעבודת הש"י:

As is stated regarding the virtues of one who engages in Torah lishmah: he merits many things, and his heart is purified for the avodah (service) of Hashem.

Citing Pirkei Avos, the Sefas Emes concludes that learning lishmah purifies the heart and readies it for true avodas Hashem.

Summary: The "highways in the heart" can be read in both directions — a person prepared within merits Torah and deveikus, and conversely, learning Torah lishmah itself engraves those very highways into the heart, purifying it for the service of Hashem.