אשר אנכי מצוך היום לעשותם, "which I command you this day to perform.⁠" Our sages in Eiruvin 22 use the superfluous word "this day,⁠" to teach that whereas performance of the commandments is incumbent upon us "this day,⁠" God does not promise that the reward will also be paid immediately, i.e. "this day.⁠" Basically, our terrestrial world is perceived a the עולם העשיה, the world of "action,⁠" as opposed to being the period in which one enjoys the reward for one's actions. Solomon already alluded to this when he said in Kohelet 9,10: "for there is no action, no reasoning, no learning no wisdom in Sheol, where you are going.⁠" His message is that it is too late to rectify after death what one has failed to rectify while alive. Once one has died one has forfeited all one's former potential. This is why one has to perform all the good one can while on this earth and capable of exercising one's faculties. This is what Solomon had said in the first half of the verse we just quoted, where we read: "whatever it is in your power to do, do with all your might.⁠"
Our sages in Avodah Zarah 3 have paraphrased this thought when they said that he who labors on the Sabbath eve will have what to eat on the Sabbath, When will the one who did not trouble himself on Sabbath eve have food to eat on the Sabbath? All these considerations commit each one of us to spend as much effort to perform a so-called מצוה קלה, an easy commandment, or one that appears capable of fulfilling with little effort as that which requires great efforts on our part. We do not know which mitzvah will be rewarded more greatly. God did not take us into His confidence [else nobody would perform the מצות קלות.] Our sages in Avot 2,1 spelled this out quite clearly. We must not commit the error of applying our yardsticks to God's thinking and deciding what in His view deserves a great deal of reward and what does not. In referring to the inadequacy of our intellect, Solomon writes in Proverbs 5,6: "she does not chart a path of life, her ways are unstable; you do not know them.⁠" We must not overestimate the reliability of the conclusions we draw about what is good and bad by second-guessing God. David had a similar message in Psalms 119,14 and 16: by emphasizing the positive, i.e. the delight of accepting God's law, His teaching in preference to all riches available on this earth.