" A candle to my feet, etc." Solomon said (Proverbs 4:18), "But the path of the righteous is like the light of dawn, etc." (Proverbs 4:19) "The way of the wicked is like darkness, etc." Why are the wicked compared to someone who walks in the dark without a candle and stumbles over a stone, hits a tree in front of him, falls into a river and doesn't know where he is going? Similarly, the wicked have no light, as it says, "the way of the wicked is like darkness."
But the righteous are like someone who walks in the dark with a candle in his hand, finds a stone and avoids it, sees a tree and doesn't hit it at all. Thus, David said, "A candle to my feet, etc." (Psalms 119:105) And it is written (Proverbs 6:23), "For the commandment is a lamp, etc." And David also said, "Even if I desire and seek to sin, the words of the Torah do not let me go, if I seek to go, they illuminate before me."
Therefore it is said, "a candle to my feet, etc." "I have sworn and I will fulfill it." David would take two rewards - the reward for the oath and the reward for the commandment. He swore to take the reward for the lulav and the reward for making a sukkah, and he would take the reward for the oath and the reward for the commandment of sukkah, lulav, tzitzit, tefillin, and circumcision. Therefore it is said, "I have sworn and I will fulfill it, etc."