To the front is the commandment:
Hear O Israel, G-d is our G-d, G-d is one
(Deuteronomy 6:4).
Here, one is to contemplate the absolute unity of G-d. This means to review in his mind the reasoning that affirms his conviction that all the experiences impinging on his consciousness originate ultimately from one single source, namely G-d, and further, that G-d is the only ultimate reality, whose essence encompasses all being. The implications of this fundamental tenant of our faith are wide-ranging and profound. For if everything we experience comes from (and ultimately is) G-d, who is axiomatically the ultimate good (see Igeret HaKodesh 11), then everything must be ultimately good. The fact that some things appear to be bad is only due to our warped vision, distorted by our own idea of how things should be.
According to the sages (Zohar 1:27b, 3:179a), for example, "when one is angry, it is as if he is serving idols." This is a strong (one could even say extreme) statement. But upon reflection, what really causes anger? When things don't go my way, I get angry; the world is not conforming to my vision of perfection. But since G-d ultimately is running everything, that is tantamount to saying that I consider my version of the world better than His! "If I were running things, they wouldn't be as messed up as they are because of the way He is running things." This is a subtle form of idol worship: I am enthroning my intellect and ego and paying homage to them.
Now, of course, not everyone who gets angry consciously realizes this, but the point is that constant awareness of the unity of G-d is the surest antidote for anger, as well as a host of other psychological maladies we would all do better without.
Hence the importance of keeping this mitzvah–awareness "in front" of us at all times.
According to sages, the ineffable four-letter Name of G-d (Havayah) signifies G-d's attribute of mercy, whereas the name Elokainu (Elokim) signifies His attribute of judgment (Shemot Rabbah 3:6). Accordingly, this commandment can be interpreted: "Hear, O Israel: G-d (Havayah) [the all-merciful] is our G-d (Elokainu) [who appears to us through His attribute of judgment, yet] G-d (Havayah) is one [and know that all is but an aspect of His absolute mercy]."
This ability to see all the seemingly divergent phenomena of life as manifestations of one, absolutely merciful source, is unique to the Jewish people. True, others may be able to understand this ideal, but only the Jew, due to his unique type of soul, can–through proper contemplation and meditation–make this way of living part and parcel of his very self. Therefore, the name for the Jew used in this verse is "Israel," which (as opposed to "Jacob," the other basic all-inclusive name of the Jewish people) signifies the experience of the Jewish soul in its absolute, pure state, qua "an essential part of G-d Himself" (Tanya chapter 2). This is also why this verse-the first verse of the Shema-is the central statement of Judaism. This attitude is fundamentally what distinguishes the Jew from the rest of humanity.
Since this commandment makes the Jew uniquely aware of his Divine inner nature, it inspires him to live up to it, i.e., emulate his Creator. And since the most essential attribute of G-d is His mercy (rachamim), meditation on the unity of G-d produces feelings of mercy within the Jew. These are directed firstly to the G-dly spark within each Jew (including himself) (Cf. Tanya ch.32), to excise it from its "exile," and then to all of reality at large, to redeem it from the shackles of its "exile" by spreading the awareness/consciousness of G-d.
