Thank you Reach4 and jadnashua very much for the comments.
After reading your suggestions, first, I tried tightening the packing nut with the wrench, and I could turn the packing nut around 1/2" to 3/4" inch. And it stopped the water dropping. I am so surprised that the packing nut was loose. I just assumed that there was no problem with packing nut tightening. As jadnashua mentioned, I think the water stopping impact loosened the packing nut over 2 decades, and it caused the leaking.
Reach4, I thank you very much for suggesting Faucet Packing Cord PTFE and the black graphite-containing cord. I will keep that in mind for the future.
jadnashua, I thank you very much for suggesting the water hammer arrestors. I heard it before vaguely but never thought of using in this kind of situation. Since I had this problem, I will consider adding it to this shut-off value. The quick internet search suggests putting this on to the dishwasher water line, toilets, and clothes washer, but I never saw this installed. Are there really WM specific shutoffs that would turn both off with one lever? My machine is whirlpool estate brand, almost 30 years, and just for my edification, could you point a link to this?
Thank you very much.