They laughed because God expected them to believe it anyway. They laughed because God seemed to believe it. They laughed because they half-believed it themselves. They laughed because laughing felt better than crying. They laughed because if by some crazy chance it just happened to come true they would really have something to laugh about, and in the meantime it helped keep them going.
Faith is laughter at the promise of a child called laughter.
There are a whole bunch of different reasons to laugh, and in that passage Buechner identifies some of them. Abraham and Sarah laugh out of recognition of the ridiculousness of what God is saying, the ridiculousness of the belief God is asking of them. They laugh because they love God enough, and each other enough, to want to believe it. They laugh because laughing heals them. One of the reasons Buechner can understand why they laughed, or that you or I can understand it, is because we've laughed ourselves.