R' Moshe Shilat
The Tanya (in Torah Or, portion of Vayechi) explains that prayer is a process of forming a link between a Jew and the Holy One, Blessed be He. He compares the elements of the process to four sons of Yaacov: Reuven, Shimon, Levi, and Yehuda.
These four sons, the first ones, are an outburst of a lasting and gradually developing relationship between Leah and Yaacov, which is an allegory for the link between the community of Yisrael and its husband, the Holy One, Blessed be He. After the birth of the first four sons, it is written, "She stopped giving birth" [Bereishit 29:35]. The sons born later on are part of a different series.
Reuven – Love
Our link to the Holy One, Blessed be He, begins with basic fear due to the acceptance of the yoke of heaven, but its most important experience takes place when love appears.
Reuven is the first son. The result of the first relationship is love. The name means, "See, a son." A son has been born to us, look at him. Looking is an expression of love, the lover and the beloved look directly at each other and that explains everything. Therefore, in Shir Hashirim – the song of love between G-d and the nation of Yisrael – there are many images that are related to vision and the eyes.
The high point of the labor of praying is seen in the first paragraph of the Shema. Here the emotion of love is openly revealed with great power – "with all your heart, with all your soul, and with all your possessions" [Devarim 6:5].
Shimon – Fear
After the advent of love, the element of fear is also necessary. This is not only the basic fear that is the basis for everything else but a great and powerful fear. The Holy One, Blessed be He, is not only close to us and our lover, He is also exalted and above us. He is way beyond us, and an honest link to Him requires fear too, a fear of the exalted that is so far above. In the second paragraph of Shema, we distance ourselves somewhat, we hear a warning: "Be wary lest your hearts will turn away" [Devarim 11:16]. This passage begins with the words, "Let it be, when you listen" [11:13]. As opposed to viewing, which brings the two sides closer, listening can take place from a distance. "G-d, I have heard Your news and I am afraid" [Chabakuk 3:2].
Shimon was the second son. After the love symbolized by Reuven – Re'u, see – we are told, "G-d has heard that I am despised" [Bereishit 29:33]. The Tanya explains this in a positive way, a feeling of distance from G-d and fear of Him, something that creates a different type of link to the Holy One, Blessed be He.
Levi – Joining Together
After strong feelings of love followed by fear, a person might be somewhat confused. How can we approach G-d in a practical way? The essence of serving G-d is to accompany Him – to do what He wants and to study His Torah. The emotions of love and fear are an introduction to the main labor of joining together with G-d. Leah says, "This time, my man will join me" [29:34].
The blessing after Shema, "Emet V'yatziv" – true and stable - corresponds to Levi. In the blessing we repeat the fact that the Torah and the mitzvot are what is important, and that we agree to be involved with them and to follow their path. After a person has acquired love and fear, he or she must join G-d in practical acts and not remain with mere emotions. The way to accompany G-d is to study Torah and observe His mitzvot.
This is expressed most strongly through the exalted tribe of Levi, which has dedicated its entire life to G-d and to practical study of the Torah.
Yehuda – Insignificance
The ultimate link between us and G-d is absolute clinging, neither feelings nor actions but being together as one. This is the revelation of the full link between us and G-d, revealing the "light that surrounds all of the worlds."
Clearly there is no hope for this without the prerequisites of love, fear, observing the mitzvot, and a large measure of Torah study. When these elements are present, a person can reach a state of absolute insignificance. This happens during the Shemona Essrei prayer, when a person negates his entire personality, in silence and with eyes tightly closed. "This time I will thank G-d" [29:35]. I will thank Him not only in the sense of thanking but also in the sense of confession and insignificance of my entire self as related to G-d, and clinging to Him completely.