It is noteworthy that Rashi - at the beginning of the parshah - quotes several statements from Chazal which explain that Hashem commenced His communication with Moshe (1:1) in a loving manner, that Moshe was specially privileged to be in close contact with God such that no one else could hear God's voice when it reached Moshe, and that God's communication with Moshe reflected the favor of Bnei Yisroel in Hashem's eyes. Divine speech with Moshe had commenced much earlier. Why does Rashi elaborate upon its nature at this juncture? Why does the Torah use the term "Vayikra", indicative of God's adoration, at this point?
It would seem that we are being taught a lesson about how to relate to and serve Hashem. Formal Avodah - which is first presented in Parshas Vayikra - must be based on each individual's commitment to and relationship with God. Communal service, such as that of the Kohanim on behalf of the larger public, can only come after each individual has already set forth a desire to cleave to Hashem and be in communion with Him. This is why Parshas Vayikra must precede Parshas Tzav. So, too, the Chazaras Ha-Shatz, in which the chazzan recites Shmoneh Esrei on behalf of the tzibbur as a whole, must follow the private Shmoneh Esrei, as an individual connection to God must be established in order for one to be meaningfully represented in formal public Avodah.
It is for this reason that Hashem's private communication with Moshe is so important as a preface to Parshas Vayikra, and that God's communication was indicative of a special love, for we are being taught that a private attachment to God - individual Avodah - is a prerequisite for formal public service. Just as korbonos (sacrifices) without inner feelings of repentance, love, supplication, etc. on behalf of the donors are meaningless, public Avodah without the prior commitment of each member of the tzibbur is sorely lacking. (It may very well be that the requirement for inner feeling in any Avodah was the primary intent of Chazal in depicting the favor of the Jews in God's eyes upon communication from Hashem, such that favorable standing and attitude are necessary for any korbon service.)
May we strengthen our private, individual commitment to Hashem, and may the entire Knesses Yisroel merit the return of Avodah in its fullest sense.