It states (16:2) 'ה עצרני נא הנה אברם אל שרי ותאמר מלדת, "And Sarai said to Avram, 'Behold Hashem has restrained me from bearing children...'"
The Midrash says that Sarah said, “I know why I don't have children. It isn't as people say that I need a kameia (amulet) or a refuah. I don't have children because 'ה עצרני מלדת, Hashem has prevented me from bearing children." She understood that it was bashert, that it was Hashem's plan.
The Sifsei Tzaddik writes, "This teaches us that we shouldn't attribute [what occurs in life] to nature, only to the Master of the world." It is easy to blame people, nature, and the like, but the wise and the righteous know that everything is Hashem's decree.
The Mishnah (Tamid 1:2) discusses the goral that the kohanim drew daily to determine which kohanim would participate in the avodah. The Mishnah states, שזכה מי זכה, "The one who won the goral, won." The Tiferes Yisrael asks that these words seem superfluous. Certainly, the kohen who won the goral was the one who merited to do that part of the avodah.
He answers that the Mishnah is saying זכה the kohen who wins the goral, שזכה מי, is the kohen that Heaven determined was worthy and fitting to do this avodah. In other words, he didn't win by chance. He won because this was Hashem's plan and decision.
Someone said to the Chazon Ish zt'l, "I want to play the lottery today. Please tell me which numbers to choose." The Chazon Ish replied, "It isn't the numbers that win the lottery; it is the person. If you are destined to win, the numbers you choose will appear in the lottery. If you aren't the person destined to win, no matter which numbers you choose, those numbers won't show up at the goral."
The same applies to all areas of life, including the parnassah one does, one's mission in life, the good times, and the not-such-good times. They are all determined from Above. They don't happen by chance. Therefore, Sarah said מלדת 'ה עצרני; she understood that she didn't have children because this was Hashem's decree and not due to some natural matter.
The Midrash (Bereishis Rabba 39) tells a story of a traveler who saw a house on fire. He asked, "Could it be that this house has no owner?" Just then, the owner peeked out from the window and said, "I am the master of this house." Similarly, Avraham looked at the world and the disasters that occur and asked, "Could it be that this world has no leader?" Hashem peeked at him and said לך לך. Hashem told him, "I am the master and the leader of the world."
Avraham believed in Hashem before. Some Midrashim say that Avraham believed in Hashem from age three. The Rambam says that he believed in Hashem from age forty. But now, when he was seventy-five, Hashem taught him that even when there is strife and suffering in the world, even then, everything is ordained from Him.
