Question: What is special about fear that only together with it will the state of love be perfect?
Answer: The fact is that we are egoists and love only ourselves. Therefore we must add fear to where we turn with love.
Each of us should understand that even if he loves someone, it is still desirable that fear and anxiety join this love: What if I do not love enough? What if I do something for him that will make him neglect me? It is not because your egoism is afraid of it, but because you do not want such a relationship between you.
From the Daily Kabbalah Lesson 2/11/24, Writings of Baal HaSulam “Joy with Trembling”