ואהבך וברכך והרבך וברך פרי בטנך ופרי אדמתך דגנך ותירשך ויצהרך... אשר נשבע לאבותיך וכו' (ז-יג)
The Torah guarantees that when the Jewish People are listening to the word of Hashem and following His precepts, He will fulfill the oath that He promised to our ancestors, and we will reap the benefit of His blessings. But if Bnei Yisroel are doing as they should and keeping Torah and mitzvos properly, shouldn't we be rewarded on our own merit? Why must we only see blessings due to the fulfillment of Hashem's vow to our forefathers?
The Noam Elimelech, R' Elimelech of Lizhensk ZT"L provides a profound response. Chazal teach: "אין מזל לישראל" - "Yisroel has no (existence with) destiny." Unlike the other nations of the world, Klal Yisroel operates under the direct protection of the Almighty Himself. However, there are three things in this world that must be provided in order for the world to exist, that has nothing to do with merit or reward: "בני,חיי,ומזוני" - "Children, Health and Sustenance." All the world benefits from these things because they are necessary components of continued life and existence.
For Bnei Yisroel, though, who do not operate under the normal terms of Mazel and human preservation, we can only receive these three all important elements of life, due to the merit and grace of our ancestors. Thus, the posuk states that in fulfillment of the ברית - covenant, that Hashem swore to our Avos, He will bless us with three things: "והרבך" - "Multiply you," which corresponds to "חיי" - "Life"; Secondly, "פרי בטנך" - "Fruit of your womb," which correlates to "בני" - "Children"; Thirdly, "דגנך ותירשך ויצהרך" - "Your grain, your wine and your new oil," which refers to "מזוני" - "Sustenance." We must rely on our זכות אבות.
ואתנפל לפני ה' כראשונה ארבעים יום וארבעים לילה לחם לא אכלתי ומים לא שתיתי וכו' (ט-יח)
Chazal tell us that just as one must bless Hashem after he has eaten and is satiated, so too, must one bless Hashem when he is hungry. Reciting a ברכה before eating is known as a סברא חשובה - superior logic, which according to some meforshim, may be deemed a דין דאורייתא -Biblical Law. (:עיין ב"ק מו)
A young chasid once came to R' Yechezkel of Kuzmir ZT"L with a kvittel in hand. The Rebbe had just sat down to eat his morning breakfast and the young man stood by respectfully observing as he ate. First, the Rebbe took a spoonful and pronounced a loud beracha, "שהכל נהיה בדברו". Then, he continued eating and when he was finished, he made a beracha acharona, to which all those present responded with a hearty Amen.
At that point, the chasid approached R' Yechezkel, kvittel in hand, and laid it on the table in front of the Rebbe. R' Yechezkel looked up at the young man and began shaking his head. "You are not like your father," he intoned balefully.
"You are like vinegar mixed inside a barrel of wine!"
Totally mystified by the Rebbe's words, the young chasid did not know how to respond. The Rebbe explained: "Your father once came to me, under very similar circumstances, with a kvittel in his hand. I was just sitting down to eat my morning meal, just as today, and I recited a loud beracha 'שהכל נהיה בדברו'. Immediately, your father raced out of the room, forgetting all about his kvittel.
"I asked him about this later and he replied, 'When I heard the Rebbe say 'שהכל נהיה בדברו', I realized that there was nothing more for me to request. Because 'שהכל נהיה' - everything will be taken care of - 'בדברו' - by the word of Hashem, and it was not my place to ask for more!'"