Lech lecha and Vayeira contain two similar reactions to the news that Avraham and Sarah would have a child. Regarding Vayeira everyone notices when Sarah laughs, including G-d. Avraham's laughter seems somehow overlooked.
Conventional wisdom, based on Rashi (I first heard this from Rabbi Chaim Bravender), suggests there are two types of laughter. When you see someone saunter by with a flower growing from his head you laugh (Rabbi Bravender's illustration) because it’s incongruous. If someone tells you that they are going to accomplish a reasonable task - and you laugh, you're implying that they can't do it.
Sarah laughed in mockery, rhetorically asking if G-d could give her a child. Avraham laughed as one laughs at jokes involving fancy suits and banana peels. One could argue that this distinction is or is not implied by the text. Whether or not this is THE pshat, there is a lesson to be learned within this approach.
Laughter can be spiritual. If we notice things as they catch us by surprise, laughter can free us to move on. Laughter can bring joy, a holy, urgent thing. If we laugh at people, we damage our souls, their souls, and the soul of the world. Sarah may have been punished for her cynicism.
This episode speaks loudly in our times. We laugh too often in mocking ways.
In a related note, at the event that marked the release of Aron Bulman's Man In A Room With A Tallis On, Aaron's son Meir brought up Sarah's laughter. Meir said that it's rare to be able to point to an exact second when a dream comes true. Together, that day we all shared the moment of Aaron's dream of publishing a book of poetry coming true - posthumously. This brought to Meir's mind, the question of why Sarah was punished for laughing at a crazy notion. She was a woman beyond the age of childbirth. Strangers approached her and said she'd have a baby. Imagine (Meir suggested) that a homeless guy in Grand Central Station tells you that a lifelong dream of yours is going to come true. The thing is he said, is that if you never gave up on your dream, then you'd smile when you heard the idea. And you'd top the smile with something like - "From your mouth to G-d's ears." Avraham laughed a joyful laugh as he recaptured his dream. Sarah had sadly let her dream go.
May we be blessed to laugh as Avraham did, in joy. May we blessed to always hold on to our dreams. And may we be continuously blessed with happy surprises from G-d.