In "Word Focus" Glenn McClure writes:
The first word Jesus used was good. There are two words in the Greek that can be translated “good” — kalos and agathos. The word used here is agathos, and Jesus' use of it is very significant. In secular Greek agathos means serviceable and good — the good which evokes a state of well-being, whether intellectual, material, religious, or moral. Kalos embraces all these meanings as well as adding the idea of beauty, of loveliness, of graciousness, of winsomeness — that which appeals also to the eye. But why did Jesus use agathos instead of kalos in this parable? Because the result of its activity is essential, that good which is by its very root and character of a moral nature was used as a quality that is the foundation for righteousness. It is that good which preserves and supports, that which does more than justice demands (it is sometimes translated “generous”). It is that good which characterizes one who can be entrusted with a responsibility that will be executed to please his master. Agathos was used because the root or character of the person will be judged.
Source: Building Christian Relationships: Adult Teacher's Insights, page 16.