Sarah was willing to give up her love for, and her life with, Abraham, to be separated from him forever–but only when that separation was merely physical.
What she was not willing, or able, to countenance, was spiritual separation.
This is where the special essence of the Abraham-Sarah relationship is revealed, and this is why the nation of Israel has two parents: Abraham and Sarah together.
It is no accident that this relationship echoes that between Adam and Eve.
Abraham and Sarah are the historical-ideological-spiritual fathers of the nation, just as Adam and Eve are its biological progenitors, the two fundamental elements of the human species.
This is why Abraham and Sarah saw themselves (and are thus seen by future generations) not as a couple raising a family, but as people building a society, realizing an ideal: parents of a nation.
To this day, converts to the Jewish faith are called "sons of Abraham," and the women among them "daughters of Sarah," because conceptually–and, indeed, halachically–Abraham and Sarah are ideological ancestors of the Jewish nation, and all who join that nation are their children.
Abraham and Sarah see themselves as leaders, forging a new road, a new worship of the Lord; as guides of a nation, diverse and yet united.
–Rabbi Adin Steinsaltz
From Biblical Images by Rabbi Adin Steinsaltz