The 3rd mitzva is that we are commanded to love G‑d (exalted be He), i.e. to meditate upon and closely examine His mitzvos, His commandments, and His works, in order to understand Him; and through this understanding to achieve a feeling of ecstasy. This is the goal of the commandment to love G‑d.
[We can see that meditation is the way to create this feeling of love1 from] theSifri: “From the statement, ‘You shall love G‑d your Lord’2, can I know how to love G‑d? The Torah therefore says, ‘and these words which I command you today shall be upon your heart’3; i.e. that through this [meditation about His commandments] you will understand the nature of ‘the One Who spoke, and thereby brought the world into being.’ “
From this it is clear that meditation will lead to understanding, and then a feeling of enjoyment and love will follow automatically [since the second verse explains the way to reach the goal of the previous verse.]
Our Sages also said that this mitzvah includes calling out to all mankind to serve G‑d (exalted be He) and to believe in Him. This is because when you love a person, for example, you praise him and call out to others to draw close to him. So too, if you truly love G‑d — through your understanding and realization of His true existence — you will certainly spread this true knowledge that you know to the ignorant and the foolish.
[We see that this mitzvah includes spreading love for G‑d to others from] the Sifri: ” ‘You shall love G‑d,’ i.e. make Him beloved among the creatures as your father Avraham did, as it is written, ‘The souls that he made in Charan.’4″
The meaning of this Sifri: Avraham, as a result of his deep understanding of G‑d, acquired love for G‑d, as the verse testifies, “Avraham, who loved Me”5. This powerful love therefore caused him to call out to all mankind to believe in G‑d. So too, you shall love Him to the extent that you draw others to Him.