Pirkei Avot 2:9

"Rabban Yochanan ben Zakkai received the tradition from Hillel and Shammai. He used to say: 'If you have studied much Torah, do not take credit for yourself, because that is what you were created to do.'"

This perek, coming after the previous one from Hillel, is at first surprising.  The previous perek says that study of Torah gives life, wisdom, and the life of the World to Come.  If this is so, why should one who has studied much Torah not take credit for it?  Freedom of choice is at the center of moral outlook of the Torah (in it, Hashem says "I set before you good and evil, life and death; choose good that you may live"; in last week's parsha, Er dies because he is evil, presumably based on his actions).  If we choose to do good things, why should we not take credit for it?

If we think about how to reconcile the two pirkei, what becomes apparent is that smugness is inconsistent with the moral outlook of the Torah.  It is unseemly and even illogical to take credit for doing what we are created and commanded to do. If we have studied well, we come away from study with the humility that comes from the sense that we are doing what we are meant and commanded to do.  One is reminded of the difference between charity and tzedakah.  Charity comes from the Latin "caritas," which is related to the heart; the sense is that is comes from good inclinations in the heart, and therefore reflects the existence of those inclinations in the giver.  The motivation to give tzedakah comes from the commandment that we give tzedakah, and ultimately from love or awe of Hashem.  As we are told in the previous perek from Hillel, performing the commandment to give tzedakah may give us the sense of peace that comes from honoring the mitzvah of tzedakah, but it should not give rise to smugness, because we are just doing what is expected of us.

In a larger sense, in a moral outlook based on mitzvot, there is no room for smugness.  Smugness implies that we are better than others because we do more good things.  However if we are created and commanded to live a certain way, living in that way is not merely an embellishment of our personality, but it enables us to actualize our purpose in this life and maybe the next.  Failing to live that way, which we all do from time to time, is a failure to live up to our purpose.