The verse shema Yisroel teaches us the mitzvah (imperative) and principle of Hashem's unity, His oneness. This verse is immediately followed by the mitzvah of ahavas Hashem (love of God). Let us, b'ezras Hashem, try to gain a measure of understanding into each of these mitzvos. This understanding will in turn, im yirtse Hashem, afford us a measure of understanding into the relationship between these two mitzvos.
On its most basic level the principle of shema Yisroel teaches that Hashem is one in an absolutely simple sense. He is not, God forbid, one of two or more (there is only one Hashem); He is not one composed of parts, etc. The principle of shema, however, is even more profound than already described. Rav Soloveitchik zt"l summarizes the Rambam's (Maimonides') exposition of shema as follows. "Maimonides formulates the idea of existing in God. Since He is the absolute Being, no other existence is possible without sharing His Being. ... The world was created as a separate substance (Judaism categorically rejects pantheism, as the Rav subsequently explains in this passage, MT), but not, however, as a separate existence. Creation is as act of tolerance on the part of God; He bestowed grace upon something by allowing it to share His reality" (Worship of the Heart, pp.126-7). We exist not only because of Hashem, but through Hashem.
Rambam describes ahavas Hashem as a state of being totally preoccupied with the love of Hashem. The love of Hashem completely dominates a person's consciousness. Accordingly, Rambam writes, "It is known and certain that the love of God does not become closely knit in a man's heart till he is continuously and thoroughly possessed by it and gives up everything else in the world for it; as Hashem commanded us, 'with all your heart and with all your soul'" (Hilchos Teshuva 10:3,6). In other words, to attain ahavas Hashem one must devote all one's energies to that sublime goal. Any and all other independent interests must be put aside. Ahavas Hashem requires exclusiveness.
The relationship between the mitzvos of shema and ahavas Hashem is now apparent. The mitzvah of shema establishes and teaches the principle of Hashem's exclusiveness. Since Hashem exists absolutely, only He exists. We exist through Him. This obligates us in ahavas Hashem: to put aside any and all independent interests and focus exclusively upon Hashem.
The attribute of ahavas Hashem is a lifelong quest. A person obviously can not attain such lofty heights overnight. But it is critical that we appreciate that the sacrificing of interests unrelated to avodas Hashem (service of God) is a sine qua non for the pursuit of ahavas Hashem. Consider the following mashal (analogy). A bachelor has time to pursue many hobbies. When he assumes familial responsibilities, out of necessity and love for his family, he curtails his hobbies. If we genuinely want to pursue ahavas Hashem, we must be ready to make the necessary (and infinitely worthwhile) sacrifices.