Even though he is delighted with his first son, why is Wang also fearful?

Wang was worried his good luck would end.

"It seemed to him as he walked into the sharp sunshine of the dusty street that there was never a man so filled with good fortune as he. He thought of this at first with joy and then with a pang of fear. It did not do in this life to be too fortunate. The air and the earth were filled with malignant spirits who could not endure the happiness of mortals, especially of such as are poor."