Abraham and Sarah had every earthly reason to be joyful when their firstborn son was born. They were very old yet God performed a miracle and enabled these faithful saints to enjoy a newborn baby when it seemed impossible. But their joy was not founded upon earthly reasons. They laughed on this great day because their joy was found in their Lord. They were joyful in God's Word, God's work in them and a faithful obedience that brought great joy.