The Meshech Chochma (based on a Chazal) explains that these two parshiyos are speaking to two different groups of people. Klal Yisrael is blessed with people who can devote themselves to the ideal (and we should recognize that it is an ideal) of complete immersion in Torah study 24x7. Klal Yisrael is also blessed with people who do mitzvos and live k’halacha, but who are not so immersed in talmud Torah. The first parsha of shema speaks to the Torah-only ideal. Here, the Torah demands, “b’chol m’odecha,” 110%. All a person’s wealth, all a person’s abilities, need to be dedicated to G-d. The same demand cannot be made on the masses who follow the second route. Similarly, the promise/threat of reward/punishment appears only in the second parsha and not the first is because it is the masses, not the Torah-only dedicated scholar, who need these extra incentives. If one is devoted to full time Torah study, there is the promise of “ohr she’bah machziran l’mutav,” that even if one goes astray, Torah study itself has the power to draw a person back. No other incentive in needed.
The gemara (Brachos 35) quotes a machlokes between R’ Shimon bar Yochai and R’ Yishmael as to how to deal with the conflict between the need to study Torah and the need for parnasa. R’ Yishmael holds that a person needs to strike a balance – to learn when it’s time to learn, and to plant and harvest (or practice law, medicine, accounting, etc.), “v’asafta deganecha,” when it’s necessary to do other things. RSHb”Y disagrees and holds that a person should aim for Torah-only. What does RSHb”Y do with the pasuk of “v’asafta deganecha?” He answers that this pasuk is speaking about those who fail to follow “retzono shel Makom.”
Tosfos on the spot asks: how does this answer make sense? That pasuk appears in our parsha in the context of the reward for those who follow mitzvos!
Based on the Meshech Chochma’s approach, the explanation is clear. It is precisely the fact that the pasuk appears in our parsha, the second parsha of shema, and not the first, which indicates that it is speaking to the less than ideal state. Relative to the high standard of Torah-only that the first parsha speaks to, this second parsha is not the “retzono shel Makom.”
The gemara (Shabbos 10) tells us that RShb”Y would not interrupt his learning to daven, but the gemara concludes that this practice should be followed only for those like himself who were completely dedicated to learning. The rule of “osek b’mitzvah patur min ha’mitzvah” normally doesn’t apply to talmud Torah – you wouldn’t say someone immersed in learning does not have to stop to take a lulav, hear shofar, etc. (Parenthetical question: why?) Why then should it apply to tefillah? Why did RShb”Y not stop learning to daven? The answer might be that the mitzvah of tefilah stems from the words “l’avdo b’chol l’vavchem” that appear in the second parsha of shema. RShb”Y held that this parsha is speaking only to those who are not dedicated full time to talmud Torah, who are not living the ideal of Torah-only. For those like himself who are, there is no mitzvah.
Why do Chazal describe the first parsha of shema as kabbalas ol malchus shamayim and the second parsha as kabbalas mitzvos? The second parsha mentions the mitzvah of ahavas Hashem, but so does the first. It mentions the mitzvah of tefillin, but so does the first. Same for mezuzah. What’s the difference? The Shem m’Shmuel answers that the difference is this mitzvah of “l’avdo…” that only appears in the second parsha. If you recognize what malchus shamayim is all about, then it’s not “avodah” and you don’t feel like an “eved.” That’s a notch down, it’s kabbala sol mitzvos, but it is not the full kabbalas ol of the first parsha.
The Torah promises that in Eretz Yisrael, “lo b’miskanus tocahl bah lechem,” (8:9) you won’t be eating bread like a poor person. Ksav Sofer writes that the point is not that we will be eating prime ribs. The point is that we will be eating bread, but not out of poverty – we will be eating bread because “pas b’melach tochal,” the way to succeed in learning is to be happy with less and not indulge in luxury. A person can do without by choice, not only by necessity. Making the choice to do so is part of the commitment to the Torah-only ideal. The Torah continues and promises that even if you are eating just plain bread, “v’achalta v’savata,” you will be just as satisfied as if you had the prime ribs, “u’barachta es Hashem Elokecha,” you will thank Hashem.