Guarding The Lighter Mitzvos

ובמדרש תנחומא שהי' דוד מתפאר הנחמדים מזהב כו' גם עבדך כו' בשמרם עקב רב

And in the Midrash Tanchuma it is brought that Dovid HaMelech took pride, as it says, "They are more desirable than gold..." (Tehillim 19:11), and "Also Your servant is careful in them; in observing them there is great reward (eikev)" (Tehillim 19:12).

Dovid HaMelech praised the mitzvos as being more precious than gold, and added that he himself guards them, for in their careful observance lies great reward.

פי' אחר שמזכיר מקודם מדרגות עליונות ואע"פ שידע מכל זה מ"מ הי' שומר המצות קלות ועי"ז הי' נזהר בהם פי' בהחמורות כנ"ל:

The explanation is that after he first mentions the lofty levels, even though Dovid HaMelech was aware of all this, nevertheless he was careful to guard the lighter mitzvos, and through this he was scrupulous in them, meaning in the weightier ones, as explained above.

Even though Dovid HaMelech had attained the highest spiritual levels, he was still meticulous in keeping the seemingly minor mitzvos, and this very care brought him to be scrupulous in the weightier ones as well.

Summary: The Sfas Emes draws on the Midrash Tanchuma, where Dovid HaMelech takes pride that the mitzvos of Hashem are more desirable than gold, and that he, as Hashem's servant, guards them carefully, for in observing them there is great reward. The Sfas Emes explains that although Dovid HaMelech had reached the loftiest spiritual heights, he did not let that lead him to overlook the lighter mitzvos. On the contrary, precisely because he was careful with the small mitzvos, he came to be scrupulous in the weightier ones. This teaches that true avodah requires guarding even the seemingly minor commandments, and through such care one is brought to greater heights in all of one's service of Hashem.