Commenting on וַיְהִ֕י כַּאֲשֶׁ֥ר הִקְרִ֖יב לָב֣וֹא מִצְרָ֑יְמָה וַיֹּ֙אמֶר֙ אֶל־שָׂרַ֣י אִשְׁתּ֔וֹ הִנֵּה־נָ֣א יָדַ֔עְתִּי כִּ֛י אִשָּׁ֥ה יְפַת־מַרְאֶ֖ה אָֽתְּ׃ Rashi writes that מדרש אגדה: שעד עכשיו לא הכיר בה מרוב צניעות שבשניהם. Unbelievable -- Avraham never looked at his wife until that point? He didn't know that she was beautiful?
R' Yehuda Leib Sorotzkin explains that when we look at our spouse, we take the whole person into account -- their personality, their behavior, everything about them. It's the whole package that makes him or her seem beautiful to us.
Until this point, when Avraham looked at Sarah, of course she appeared beautiful to him, but that was because when he saw her he was also looking at the pnimiyus of who she was, the whole picture, including the depths of her personality.
This was the first time that Avraham took account only of the chitzoniyus, the way the Mitzrim looked at people, and became aware that even on that level alone Sarah was beautiful.