At first I thought this was just downright hilarious for the look on his face. But now I think it seems related to God. This thing that he has made is bowing down to him. Not only is he delighted by the finished product, but that this "product" is choosing to love and worship him. So he's smiling in wide eyed wonder, and his joy is so heartfelt that he is almost about to laugh. That precipice of determined action, the deep breath before the plunge is greatest, when he has determined that he is about to instinctively laugh out loud before he does it. What follows after, though not diminished by it, is merely the inevitable execution of his innocent reaction.