Although fear and love are also among the 613 mitzvot,
וְהִנֵּה, אַף דִּדְחִילוּ וּרְחִימוּ הֵם גַּם כֵּן מִתַּרְיַ"ג מִצְוֹת,
they are nevertheless described as [mere] wings for other mitzvot,
אַף עַל פִּי כֵן, נִקְרָאִין "גַּדְפִין",
because the goal of love is the service of G-d resulting from this love.
לִהְיוֹת, כִּי תַּכְלִית הָאַהֲבָה הִיא הָעֲבוֹדָה מֵאַהֲבָה,
Its purpose lies not in itself but in its role as motivation for serving G-d via the mitzvot. For this reason, it is likened to “wings,” which are secondary to the bird itself.
Love without “service” i.e., a love that is not a means to an end but an end in itself is a “love which experiences delights,” a supremely high level of love in which one delights in G-dliness.
וְאַהֲבָה בְּלִי עֲבוֹדָה, הִיא "אַהֲבָה בְּתַּעֲנוּגִים", לְהִתְעַנֵּג עַל ה'
This is in the nature of the World to Come and thus constitutes reward.
מֵעֵין עוֹלָם הַבָּא וְקַבָּלַת שָׂכָר,
I.e., such love for G-d is actually a foretaste and part of the reward to be given in the World to Come; it does not in itself represent service of G-d.
But it is written, “Today to do them (i.e., ‘today’, this life in the time of action and service) and tomorrow (in the World to Come) to receive their reward.”17 Thus, in this life, the time of service, the love that leads to service is the love most prized.
וְ"הַיּוֹם לַעֲשׂוֹתָם", כְּתִיב, וּ"לְמָחָר לְקַבֵּל שְׂכָרָם".
He, however, who has not attained to this level of savoring a foretaste of the World to Come and has not reached the level of “a love which experiences delights,”
וּמִי שֶׁלֹּא הִגִּיעַ לְמִדָּה זוֹ, לִטְעוֹם מֵעֵין עוֹלָם הַבָּא,
but whose soul yet yearns and thirsts for G-d and goes out to Him all day long,
אֶלָּא עֲדַיִין נַפְשׁוֹ שׁוֹקֵקָה וּצְמֵאָה לַה' וְכָלְתָה אֵלָיו כָּל הַיּוֹם,
and he does not quench his thirst for G-dliness with the “water” of Torah that is in front of him—such a person is comparable to one who stands in a river and cries: “Water, water to drink!”
וְאֵינוֹ מְרַוֶּה צִמְאוֹנוֹ בְּמֵי הַתּוֹרָה שֶׁלְּפָנָיו – הֲרֵי זֶה כְּמִי שֶׁעוֹמֵד בַּנָּהָר וְצוֹעֵק: "מַיִם! מַיִם לִשְׁתּוֹת!",
Thus, the Prophet laments over him: “Ho, all of you who thirst, go to the waters!”18 on which our Sages comment19 that “water” refers to Torah.
כְּמוֹ שֶׁקּוֹבֵל עָלָיו הַנָּבִיא: "הוֹי כָּל צָמֵא לְכוּ לַמַּיִם".
Surely the Prophet is not addressing (as the simple meaning of the words would indicate) one who thirsts for Torah, for such a person will surely quench his thirst and study Torah without the Prophet’s exhortation. Clearly, then, these words are addressed to one who thirsts for G-d, and the Prophet tells him that he must quench the thirst of his love by studying and practicing the Torah.
In the Alter Rebbe’s words:
For in its simple meaning, the verse is incomprehensible:
כִּי לְפִי פְשׁוּטוֹ אֵינוֹ מוּבָן,
he who is thirsty and desires to study [Torah] will surely do so of his own accord.
דְּמִי שֶׁהוּא צָמֵא וּמִתְאַוֶּוה לִלְמוֹד, פְּשִׁיטָא שֶׁיִּלְמוֹד מֵעַצְמוֹ,
Why must the Prophet cry over him, “Ho”? Clearly, then, the verse refers to one who loves G-d and thirsts for Him.
וְלָמָּה לוֹ לַנָּבִיא לִצְעוֹק עָלָיו "הוֹי"?
Now, if love of G-d were an end in itself, the service of prayer could suffice, for it creates a love and thirst for G-d. But because the purpose of love is that it lead one to serve G-d, the Prophet exhorts us not to rest content with love itself but to study Torah and thereby quench the thirst for G-dliness and also realize the purpose of love.
This matter is discussed elsewhere at length.
וּכְמוֹ שֶׁמְּבוֹאָר בְּמָקוֹם אַחֵר בַּאֲרִיכוּת:
The Alter Rebbe previously compared love and fear of G-d to the wings of a bird; the wings are not the essential part of the bird but merely serve it, enabling it to fly; likewise, love and fear merely serve the mitzvot, which are the essential objective, by elevating them to a level where the union effected by them can be revealed.
He now raises a question: Love and fear of G-d are themselves enumerated among the 613 mitzvot; why, then, are they assigned a secondary status?