The pasuk implies that Avraham sent an agent (Yishmael) to bring the water. Rashi states that Avraham should have personally attended to the guests. As a result of his personal non-involvement, when Hashem responded to Avraham’s descendants, He also sent an agent (Moshe) to give water to them. Why is this? The act of chesed, kindness, was performed, albeit through an agent. Why then should there be negative repercussions?
The Chofetz Chaim offers two insights. First, it is always more propitious to personally attend to the mitzvah, rather than to delegate it to an intermediary. Second, it is a greater courtesy for guests to be served by the master of the household. Avraham Avinu, who exemplified the middah of chesed, was impelled to perform this act of kindness, since he was setting the standard for future generations. Because Avraham did not perform this act of kindness to the highest degree possible, his reward was similarly imperfect.
Horav Moshe Feinstein z.l., questions this. The agent whom Avraham sent was Yishmael, his son. Obviously Avraham’s intent was to educate Yishmael in the middah of chesed. Was it so wrong for Avraham to permit his son to serve the guests? Horav Feinstein asserts that the proper method of chinuch, education, is for the student/child to observe the teacher/parent involving himself in performing acts of kindness. It is not sufficient for the child to merely hear lectures from the parent about the importance of being a baal chesed; he must see this attitude modeled in his own home. A child growing up in a home in which the parents are actively involved in helping people will naturally gravitate to this way of life.
We may add that this idea applies to every area of education. Parents who want their children to study Torah must do so themselves. Children learn to respect and emulate what they feel is valued and appreciated at home. Indeed, the primary classroom is the home.
