וְעַתָּה֙ יִשְׂרָאֵ֔ל מָ֚ה יְהוָ֣ה אֱלֹהֶ֔יךָ שֹׁאֵ֖ל מֵעִמָּ֑ךְ כִּ֣י אִם־לְ֠יִרְאָה אֶת־יְהוָ֨ה אֱלֹהֶ֜יךָ לָלֶ֤כֶת בְּכָל־דְּרָכָיו֙ וּלְאַהֲבָ֣ה אֹת֔וֹ וְלַֽעֲבֹד֙ אֶת־יְהוָ֣ה אֱלֹהֶ֔יךָ בְּכָל־לְבָבְךָ֖ וּבְכָל־נַפְשֶֽׁךָ׃
And now, Israel, what doth the LORD thy God require of thee, but to fear the LORD thy God, to walk in all His ways, and to love Him, and to serve the LORD thy God with all thy heart and with all thy soul;
Zohar
"After the death of the two sons of Aaron" Rabbi Yitzchak opened the discussion saying, "Serve Hashem in fear, and rejoice with trembling" (Tehilim 2:11). It is also written, "Serve Hashem with gladness: come before Him with singing" (Ps. 100:2). These verses appear to contradict one another. We have learned that "serve Hashem in fear" means that one must first show fear and awe in every act he wishes to perform before his Master. As a result of this reverence before his Master, he will merit to serve with joy the commandments of the Torah. Therefore, it is written, "What does Hashem your Elohim require of you, but to fear" (Devarim 10:12).