Today's reading in Tanya talks about fear/awe (yirah) and love (ahavah) of G!d - and it reminds me of how these fundamental ways of approaching the service of G!d are expounded in Mesillat Yesharim, one of the foundational texts in the study of Mussar.
Tanya on awe: For when the intellect in the rational soul deeply contemplates and immerses itself exceedingly in the greatness of G-d, how He fills all worlds, or when one contemplates how He encompasses all worlds, or, when one contemplates another aspect of G-d’s greatness, namely, how in His presence all creation is considered as naught—upon contemplating and immersing oneself in any of the abovementioned aspects of G-d’s greatness, the middah of awe for the Divine Majesty will be born and aroused in one’s mind and thought, to fear and be humble before His blessed greatness, which is without end or limit, and there will also be born a dread [pachad] of G-d in his heart.
Mesillat Yesharim: Fear [of G-d] - this is fear of the exaltedness of G-d, namely, that one fears before Him like he would fear before a great and awesome king, feeling abashed by His greatness before making any movement before Him. All the more so, when speaking before Him in prayer or studying His Torah.
Tanya on love: Next, his contemplation his heart will glow with an intense love of G-d like fiery flames, with a passion, desire, and longing, and a yearning soul toward the greatness of the blessed Ein Sof. This is what is meant by the term kalot hanefesh (“a consuming passion of the soul”), as it is written: “My soul longs [for You]; indeed, it faints…,” and “My soul thirsts for G-d…,” and again, “My soul thirsts for You….”
Mesillat Yesharim on love: Love - that one has implanted in his heart love of G-d, until his soul is moved to do what is pleasing to Him just like his heart is moved to do what is pleasing to his father and mother. He will be pained if he or others are lacking in this. He will be zealous for it and feels great joy in doing something of this.
Rabbi Moshe Chaim Luzzatto, who wrote Mesillat Yesharim, was a kabbalist, so I am not surprised that he and the Alter Rebbe came up with similar language to describe these - I would love to know more about how these two aspects of the service of G!d are treated in prior works that both might have read and been influenced by.