Rabbi Kaplan in Handbook of Jewish thought chapter 3 #11-18 discusses Hashem's goodness. In paragraph 18 he quotes that line from Derech Hashem and explains it according to what he just finished saying, to mean the goodness of free will and the pleasure that comes from accomplishments that recognizably come from that free will. These are his words: Since the ultimate good is God Himself, the greatest possible good that He can bestow is Himself. There is no greater good than achieving a degree of unity with God, the creator of all good. Since God desires to give man the greatest good possible, He gave him the ability to resemble Himself.

* The following is from a previous incarnation of this question and answer. *

From Rabbi Kaplan's Handbook Of Jewish Thought, chapter 2, paragraph 3.

As creator of the universe, God's existence can't depend on any of His handiwork. Judaism therefore rejects any definition of God as an abstract ethical force or social convention.

In footnote #2 he writes: "Thus, God cannot be defined as love, truth, justice, goodness, or in any other other human terms. While these are attributes of God, they are not God Himself".