(Devarim, Ibid. 13) "to love the L-rd your G-d": Lest you say: I shall learn Torah to be rich, to be called Rebbi, to receive reward in the world to come — it is, therefore, written "to love the L-rd your G-d." All that you do shall be out of love alone.
"and to serve Him": This is learning. — But perhaps it is service, literally. It is written (Bereshith 2:15) "And the L-rd G-d took the man and placed him in the Garden of Eden to serve it and to guard it." Now what "service" was there in the past, and what "guarding" was there in the past? We are hereby taught "serving" is learning, (in this instance, the "seven mitzvoth of the sons of Noach) and "guarding" is (the doing of) mitzvoth. And just as the (sacrificial) service of the altar is called "service," so, learning is called "service."