Musar על אבות 1:3

The common denominator of the concepts of שבת and משכן are that they both are symbols of עולם הבא, the World to Come. There is a baffling story in the Talmud Beytzah 16 describing that Shammai bought and ate only food intended to honour the Sabbath. Should he find a good looking animal, he would purchase it intending to eat it on the Sabbath. Should he find a superior one later, he would abandon the first one and eat it during the week so as to preserve the better one for serving on the Sabbath. The Talmud contrasts Shammai's conduct with that of his colleague Hillel whose every action is described as being לשם שמים, "Heaven oriented." Are we to infer from this that Shammai's actions were not "Heaven oriented?" What then was the difference of opinion between Hillel and Shammai which caused them to have different approaches to their daily routines? They disagreed on whether it was permissible to serve G–d in order to accumulate rewards payable in the World to Come. (This is elaborated on in Midrash Shemuel Avot 1,3.) Shammai considered serving G–d for material rewards in this world as forbidden, whereas he considered serving G–d in order to accumulate reward in the Hereafter as perfectly permissible. His colleague Hillel considered serving G–d in order to receive a reward either in this world or in the next as equally forbidden. This is what the Talmud means when describing Hillel's actions as "Heaven oriented," i.e. without thought of any reward.

There is another type of "not for the sake of the mitzva itself" (Shelo Lishma), which is the "for the sake of receiving a reward" (Avot 1:3). On this our sages said: "a man should always occupy himself with Torah and good deeds, even if it is not for their own sake, for doing so will lead to doing them for their own sake" (Pesachim 50b). Nevertheless, he who has not yet reached from "not for their own sake" (shelo lishma) to "for their own sake" (lishma), is still far from reaching his Shelemut (wholeness/perfection).