Nobody will know that you are going to anoint David. Then, once you get there, you can do it quietly so that Shaul won t know anything about it. G-d doesn t want to do a miracle, even to save Shmuel from the dangers of his mission. Besides that, miracles lead people to a mistaken idea of nature. The truth is that there is no real difference between miracles and nature.
Nature is just miracles that happen all the time.
But since it happens everyday, so it becomes old hat. Imagine hearing about a machine that could go on for years without rest and without servicing. The human heart is a miracle, but its with us all the time, so we get used to it. The sense of wonder which reveals nature for what it really is, wears out. There was a little bit of oil left, so when she lit the candles they started to burn.