Sefer VaYikra begins with God calling Moshe into the Ohel Moed:
"He called to Moshe, and the Lord spoke to him from the Tent of Meeting, saying:" (VaYikra 1:1).
However, the Torah's standard formula for the introduction of a new subject is:
"The Lord spoke to Moshe, saying".
In order to understand this instance of Moshe needing an invitation from God, we need to examine the other instances when God called upon Moshe:
1. At the Burning Bush:
"The Lord saw that he had turned to see, and God called to him from within the thorn bush, and He said, "Moshe, Moshe!" ... "Do not draw near here. Take your shoes off your feet, for the land upon which you are standing is holy" (Shemot 3:4-5).
2. Three times at Maamad Har Sinai:
"Moshe ascended to God, and the Lord called to him from the mountain, saying:" (ibid 19:3).
"The Lord descended upon Mount Sinai, to the peak of the mountain, and the Lord called Moshe to the peak of the mountain, and Moses ascended" (ibid 20).
"The glory of the Lord rested on Mount Sinai, and the cloud covered it for six days, and He called to Moshe on the seventh day from within the cloud" (ibid 24:16).
The difference between the Burning Bush and the Mount Sinai episodes, is that at the Burning Bush, Moshe wanted to approach and God called upon him not to. While at Mount Sinai, Moshe did not approach God, so God called upon him in order to approach.
Why did God stop Moshe from approaching Him at the Burning Bush and why did Moshe not automatically approach God at Mount Sinai?
Solving this problem will help us understand why Moshe was called in this week's parsha.
At the burning Bush, God actually tells Moshe why he could not approach: "...the land upon which you are standing is holy". Moshe was not in a fitting state to have an extremely close encounter with God. Unprepared close encounters with God leads to death: "... for no one can see me and live" (Shemot 33:20).
Indeed, Yaakov was surprised that he survived his encounter with the celestial being (Bereshit 32:20) as were Gidon (Shoftim 6: 22-23) and Manoach (ibid 13:22).
At Mount Sinai, God's glory was resting in a cloud at the peak of the mountain. Moshe could not approach Him so God needed to call upon him. Once invited, Moshe could enter the cloud and have communion with God.
So too, in the this week's parsha. As the Rashbam explains (on Vayikra 1:1), Parshat VaYikra is an immediate continuation of Sefer Shemot. Thre, the Mishkan had just been completed and God's presence was resting on it. Therefore, "Moshe could not enter the Tent of Meeting because the cloud rested upon it and the glory of the Lord filled the Mishkan" (Shemot 40:35).
So, "He called to Moshe" to give him permission to enter the Ohel Moed.
Incidentally, the kohanim at the consecration of the first Temple were unfortunately excluded from this communion: "It came to pass, when the priests had come out of the holy place, that the cloud filled the house of the Lord, so that the priests could not stand to minister because of the cloud; for the glory of the Lord filled the house of the Lord" (I Melachim 8:10-11).
The Kohanim did not receive the calling and remained outside.