Rabbi Yonasan Sacks
One of the traits that the Torah attributes to Noach and Avraham is that of "temimus," or completeness. Noach is referred to as a "tzaddik tamim," a righteous, complete person. Avraham is commanded, "hishalech lefanai veheye tamim," walk in front of Me and be complete. Furthermore, the Torah tells us in Parshas Shoftim, "tamim tihyeh im Hashem Elokecha," be complete with Hashem your G-d. The Ramban counts this as one of the 613 mitzvos, with two parts: acknowledging Hakadosh Baruch Hu as omniscient and omnipotent, and seeking one's needs only from Hakadosh Baruch Hu, not through any other channel.
The Rambam does not count "tamim tihye" as one of the 613 mitzvos. This omission can be explained in several ways. According to the Terumas Hadeshen (psakim 96), the Rambam's omission is consistent with what he wrote in Hilchos Avodah Zarah (11:16). The Rambam there explains that one cannot seek out his needs through any channel other than Hakadosh Baruch Hu. This is not because other media have inherent powers. The Torah forbade them because they are all false and have no power or validity at all. The commandment "tamim tihye", then, is axiomatic - one should completely follow G-d because only He has the power to help. The Ramban himself suggests that the Rambam does not count general, comprehensive principles as mitzvos. "Kedoshim tihyu," for example, is not counted as a specific mitzva, but is rather a general approach to religious life. The same reasoning, the Ramban suggests, could apply to "tamim tihye."
Alternatively, the Rashba in his teshuvos (1:413) explains that "tamim tihye" is not a commandment, but a promise or condition. Rashi, commenting on the possuk "tamim tihye," explains "kol mah sheyavo alecha, kabel bitmimus." If a person accepts with "temimus" everything he experiences, i.e. lives up to "tamim tihe", then "az tihye imo ulechelko", i.e. then one will be "im Hashem Elokecha".
The Gemara in Shabbos (30b), based on the final verse in Koheles "ki ze kol ha'adam," explains that the whole world was created for man. The Akeidas Yitzchok, however, raises an obvious difficulty. When G-d created all other creatures, He said "ki tov," it was good. However, when G-d created man, He did not say "ki tov." If man was the reason for creation, why wasn't "ki tov" pronounced when man was created? One could suggest that the very existence of other creatures, who are not endowed with free will and evil inclination, is a fulfillment of the will of Hakadosh Baruch Hu. Human beings, however, must exercise their bechira chofshis to realize their full potential. Their creation alone is not the achievement of this goal, rather a person must always strive to improve himself, and attain excellence in his service of, and closeness to, Hakadosh Baruch Hu.
The Sefer HaChinuch (Mitzvah 2) writes that this idea is evident in the mitzva of bris milah. Hakadosh Baruch Hu created man imperfectly, and commanded him to perfect himself. The Chinuch continues, "Ki ka'asher tashlim tzuras haguf al yado," just as a person must perfect himself physically, "ken beyado lehashlim tzuras nafsho," so too one must constantly strive to attain temimus, i.e. spiritual perfection.