From International Standard Bible Encyclopedia
This is the favorite self-designation of Jesus in the Gospels. In
Matthew it occurs over 30 times, in Mark 15 times, in Luke 25 times,
and in John a dozen times. It is always in the mouth of Jesus Himself
that it occurs, except once, when the bystanders ask what He means by
the title (John 12:34). Outside the Gospels, it occurs only once in
Acts, in Stephen's speech (Acts 7:56), and twice in the Book of
Revelation (1:13; 14:14).
Because this is the title Jesus prefers for himself, it follows that it would be used in his first self-reference. Up to that point, all titles given for Jesus are provide by the author or someone else.
Because this title is used exclusively by Jesus, this feature is not unique to Mark, but true across the gospels. In all 4 gospels, the first use of "Son of Man" is by Jesus. see Matthew 8:20, Luke 5:24, and John 1:51.
Titles Jesus uses in Mark:
Using a red-letter edition of the ESV, I did a quick count of titles Jesus used (counting some that he may not have been using for himself, and some in which he is quoting from someone else) and came up with the following counts:
- Lord --- 5:19; 12:36,37
- prophet --- 6:4
- Christ --- 9:35; 12:35; 13:31
- Son of David --- 12:35
- Son --- 13:32
- Teacher --- 14:14
Total count: 10
- Son of Man 2:10; 2:28; 8:38; 9:12,31; 10:33,45; 13:26; 14:21 x2,41,62
Total count: 121
This shows "Son of Man" is the title Jesus prefers to use for himself in Mark.
Example of preference:
One notable example is when the High Priest asks Jesus if he is the Messiah using two different titles. Jesus still uses "Son of Man," even when he answers in the affirmative.
But he remained silent and made no answer. Again the high priest asked
him, “Are you the Christ, the Son of the Blessed?” And Jesus said, “I
am, and you will see the Son of Man seated at the right hand of Power,
and coming with the clouds of heaven.”
This is also the key to understanding why Jesus prefers this title in the first place. It is a clear reference to Daniel 7:13,14:
“I saw in the night visions,
and behold, with the clouds of heaven
there came one like a son of man,
and he came to the Ancient of Days
and was presented before him.
And to him was given dominion
and glory and a kingdom,
that all peoples, nations, and languages
should serve him;
his dominion is an everlasting dominion,
which shall not pass away,
and his kingdom one
that shall not be destroyed.
It seems very likely this is the reference Jesus has in mind when he uses the title "Son of Man." Herein a mysterious figure having the appearance of a man appears before the "Ancient Of Days" and is given dominion over the earth. This is certainly inline with his claims, as Mark 2:10 is a good example: So that "you may know that the Son of Man has authority on earth to forgive sins..."
1 The reason for the discrepancy between my count and ISBE is that verses 8:31 and 9:9 are not in red letters (although Jesus is the one speaking indirectly) and 13:34 is not translated "Son of Man" in the ESV.