Kabbalah zu Tehillim 100:2

"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).