Sara realizes that her being an: עֲקָרָה (unable to have children) was not happenstance; it was Min HaShamayim and Ratzon Hashem! But she didn’t just accept it as a permanent Matzav, like so many Yi'ush oriented type of people. She looked at it as a Hashgacha Min HaShamayim for her to go against her fate, and Hashem would Davka help her in being Zoche to children. So she said to her husband: Marry my maidservant, and through that Zechus I will have children as well! (Rashi).
Similarly, we have Leah Imeinu who davened against her Goral (destiny) to marry Eisav, since the older one marries the older one (Rashi). A human being is a: חֵלֶק אֱלוֹקַ מִמַעַל piece of Hashem, and is expected to make things happen by changing his destiny and overcoming “impossible” situations.
This was the tragedy of “Acher” (Elisha ben Avuya). The Gemara tells that “Acher” heard a: בַּת קוֹל Heavenly voice saying that everyone can do Teshuva except for “Acher”; this made him lose all hope, and he went off the Derech completely. Instead, he should have said (as it says in Bava Metzia): אֵין מַשְׁגִּיחִין בְּבַת קוֹל B"M 59b We don’t listen to “Bas-Kol’s” (Heavenly voices)!!! Incidentally, in that Gemara in Bava Metzia we have fascinating stories about how Chazal were strong to defend their positions.
