All human beings were created b'tzelem Elokim and one of the mitzvos of the Torah (in parshas Ki Savo) is "v'holacto bidrochov", i.e. to preserve that tzelem elokim which was implanted within us at birth by acting according to the middos of Hashem (see Rav Chessed, vol 2, page 205).
In the beginning of parshas Kedoshim the Torah repeats this idea but with a slight modification; here the Torah commands us to act in a kadosh manner because Hashem is kadosh. There is, however, a slight difference in the wording used to describe how we should act vs. the word describing Hashem. "Kedoshim", describing how we should act, is written choseir (without a vov) while the word describing Hashem - "Kodosh" - appears in the Torah moleh (with a vov). The Tannaim in the Sifroh picked up on this discrepancy and understand this to mean that we should not attempt to be on the same level of kedusha as Hakadosh Boruch Hu. His kedusha is complete (moleh) while ours can only be incomplete (choseir). What are the Tannaim driving at with this point?
About one hundred years ago a suggestion was made that perhaps the Jews should incorporate the Gospels into their Bible and thereby solve the age old problem of anti-Semitism. Achad Ha'am, who was neither a believing nor an observant Jew, published an essay rejecting the idea. He explained that the Jewish Bible and the Christian Bible simply cannot blend in together because there are several major glaring contradictions between the two. One of those glaring contradictions was regarding altruism. Judaism teaches that every man is basically selfish and as such should strive to treat other people's needs as if they were our own. But whenever there is a contradiction between the two precedence is given to our own needs (see Bava Metziah, 62A, "chayecha kodmim").
In the introduction to his major work, Shaarei Yosher, Ha'gaon R' Shimon Shkop z"l suggests that perhaps that is what the Tannaim were driving at in their comment mentioned above. Kedusha in this context refers to our doing chessed for others. The medrash points out that the Torah both begins and ends with Hakadosh Baruch Hu doing gemilus chassodim (Sotah, 14A). In addition, the medrash considers the theme of the Book of Ruth to be a demonstration of the reward that one receives for practicing gemilus chassodim. Despite this emphasis on chessed, we should not think that we should dedicate ourselves for doing chessed for others in a purely altruistic fashion - that is only for Hakadosh Baruch Hu. The mishna (Avos 1:14) quotes Hillel as telling us that we are always motivated with selfish feelings ("im ein ani li, mi li?"), but one should not define "himself" as only himself ("k'shani l'atzmi, mo ani?") Rather, we should each view our family, our community, our people, and ultimately all of mankind, as an extension of ourselves and therefore by doing for others we are really doing for ourselves.[1]
[1] See Sefer Mepeninei Horav, likutim on parsha, parshas Breishis, #10.