“Evaluate … for you do not know the reward for the mitzvos”
The Satmar Rov comments that kiyum hamitzvos is indeed predicated on the fact that Hashem does not inform us of the reward for each mitzvah. Thus, if a person will fulfill many mitzvos he can be certain that he will earn a substantial reward, because surely among the multitude of mitzvos that he performs, there are those which have a great reward. If the individual will only engage in one, or a few mitzvos, he cannot be assured of that sizeable reward because he does not know the value of each mitzvah.
The Abarbanel suggests that the Tanna is saying that we do not know whether the reward is for the fulfillment of the basic mitzvah or are we remunerated for the zeal and joy with which we perform the mitzvah.
