After hearing the blessing/promise (see meforshim) of the visitor/angels that she would have a son, Sarah Imeinu cannot restrain herself from laughter, albeit “b’kirba”, in the depths her heart. Nonetheless, Hashem immediately rebukes her for having the slightest doubt that such a promise or prophecy could come true. Yet, in last weeks’ parsha we read that Avraham also laughed when he was told that he would have a son, and surprisingly he receives no rebuke. Why the difference? The classical approach (Rashi) is to distinguish different types of laughter – the laughter of Avraham was one of joy, the laughter of Sarah was one of doubt. The Meshech Choma offers a brilliant insight based on a Rambam we discussed in the past. The Rambam writes in Into to Peirush haMishna that what Hashem privately promises a Navi may not be fulfilled – it is subject to terms and conditions, such as the Navi being required to remain at a high level of tzikdus, e.g. although Hashem promised to protect Ya'akov, Ya’akov was fearful of encountering Eisav because he thought his sins would negate the promise. Yet, continues the Rambam, nevuah, words that a prophet is instructed to publicly proclaim, are guaranteed to come to fruition (see Rambam Yesodei HaTorah ch 10 that a Navi can be tested by seeing whether his words are fulfilled). When Avraham heard the private promise told to him that he would have children, he laughed as he could not conceive of his being worthy of such a zechus coming to fruition. However, Sarah heard the promise of children uttered by a Navi – Avraham was told to change her name to Sarah (17:15-16) and tell her she would have a son. A promise proclaimed by a Navi is unconditionally guaranteed. Therefore, Sarah’s laughter was inappropriate.
(See Maharal, Gevuros Hashem ch 7 for more on this yesod fo the Rambam)