After many years of failing to have children, after having grown too old with her husband to conceive, Sarah did not expect to ever become a mother. So when the melachim — or, as she saw them, travelers — came to Avraham’s tent, promising her a son in a year, she cynically dismissed them. (The Ramban (Bereishis 18:15) notes that the phrase ותצחק בקרבה has the connotation of לעג, or cynicism. The Ramban references Tehilim 2:4 as another example: יושב בשמים ישחק ה’ ילעג למו.) The Ramban (Bereishis 18:15) claims Sarah made two missteps: Sarah, being a נביאה, should have had the spiritual sensitivity to recognize these “men” as malachim, but instead she thought they were travelers. The Ramban claims, moreover, even if she didn’t recognize them as malachim, she should still not have cynically dismissed their beracha, and instead have said, “כן יהי רצון”.
This criticism seems strange. As far as Sarah could tell, the people in her home were ordinary men — they did not deserve to be taken seriously. Moreover, Sarah herself was tortured by the fact that she could not have any children, so when these men said she would have a son in a year, she almost certainly felt that same pain all over again. Cynically dismissing their claims as impossible — as indeed they were — seems like a reasonable way for her to respond.
I think that the Ramban means that Sarah, as a tzadeikes , should have had the strength of character to confront her shortcomings. Sarah was a spiritually ambitious person: she desperately wanted to raise her children to be a part of Avraham’s bris with Hashem. Her ambitions made her vulnerable to the pain of failure — precisely because she placed so much value in having children, not being able to have children pained her. But a tzadeikis is expected not to shy away from this pain, but to overcome it. She cynically dismissed the malachim because to take them seriously would mean to admit to herself that she had not succeeded — to admit that she did not have the life she envisioned for herself.
Part of greatness is ambition and achievement: setting our sights high and pursuing our goals. But part of greatness is also accepting failure — even when that failure seems final. Seeking virtue makes us vulnerable to the pain of our disappointments. But there is also virtue in vulnerability — the power to accept these disappointments and the willingness to overcome them are the stuff of tzidkus.