Before destroying Sodom and its environs Hashem felt that it was only proper that He inform Avraham of their impending destruction: "Will I conceal from Avraham that which I am intending to do? For I am fond of him because he will educate his children and household to follow the ways of Hashem, to perform acts of charity and carry out justice, so that Hashem will bring upon Avraham that which He spoke about him" (Bereishis18:17,19). Rashi explains that Avraham commanded his offspring to guard the ways of Hashem in order that Hashem reciprocate and deliver to his family all the good that He had spoken about.
This idea seems quite difficult in light of the Mishna in Pirkei Avos (1:3). The Tanna exhorts us, "Do not be like the servants who serve their master with the intention of receiving remuneration. Rather, be like those servants who serve the master without the intention of receiving remuneration." With this in mind, why would Avraham specifically command his children to serve Hashem with the explicit intention of receiving reward?
Rav Wolbe (Shiurei Chumash, Parashas Vayeira18:19, 22:12) explains that although in one's personal avodah he should strive to serve Hashem without an eye on the paycheck, nevertheless, when it comes to educating one's children it must specifically be done with an emphasis on reward. The Rambam (Hilchos Teshuva10:4-5) writes that attaining the level of "lishma" serving Hashem with the single intention of doing His will, is a level reserved for the wisest of men and their disciples. The Rambam continues, "Therefore, when teaching children... one should teach them to serve Hashem out of fear [lest they get punished] and with the intention of being rewarded." The foundation of chinuch is educating a child to do a mitzvah "she'lo lishma" i.e. with the specific goal of receiving reward.
Later in the parsha, Rav Wolbe extends this idea even further. After Avraham successfully passed the tenth and final test of offering up his long awaited child, Yitzchok, the angel told him, "Now I know that you fear Hashem" (ibid. 22:12). Was it only now that Avraham's fear of Hashem was apparent? Had he not already passed nine other tests? Although Avraham had shown that he possessed fear of Hashem, he hadn't attained perfection in this area. Perfection means that no flaw whatsoever can be found in the middah. If at any point an ulterior motive is found, it suggests that all previous behavior might have merely been performed for that ulterior motive. Avraham's possible ulterior motive was his son Yitzchok. His entire service of Hashem might have been carried out with the intention of being rewarded with offspring who would carry his name and continue his legacy. When he brought his son as an offering, he proved to one and all that he fulfilled Hashem's commandments regardless of any ulterior motive.
Unfortunately, says, Rav Wolbe, our avodas Hashem is almost entirely based upon ulterior motives. He would say, "The boys come to Shachris because there is a Mashgiach, and I come to Shachris because I am the Mashgiach!" Rav Yeruchom Levovitz zt"l would illustrate this idea with the following example. One might be davening in his in-law's house and thinking that he heard his mother-in-law passing by, he would intensify his concentration. However, after realizing that it was only a cat, his intense concentration would suddenly wane!
Truth be told, continues Rav Wolbe, we shouldbase our avodah on a concrete ulterior motive. This will give us the drive to become bigger people. Once someone reaches a higher level he will have the ability to discard the ulterior motive, but until then it is imperative to find a good reason to become a great person. Whether the impetus is money, honor or your reputation, it's a desirable springboard so that you can be all that you can be in the army of Hashem!