Why does Jesus refer to himself as the "Son of Man"?

This is a reference to a vision in the book of the prophet Daniel:

₁₃ 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. ₁₄ And to him was given dominion and glory and 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. — Daniel 7:13-14 RSV

Jesus refers to this when He is brought before the council, as described in Mark 14:

_61b Again the high priest asked him, “Are you the Christ, the Son of the Blessed?” ₆₂ 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. — Mark 14:61b-62 RSV

See also: Luke 22:69

He also refers to it when speaking about the End Times:

Then will appear the sign of the Son of Man in heaven. And then all the peoples of the earth will mourn when they see the Son of Man coming on the clouds of heaven, with power and great glory. — Matthew 24:30 NIV

See also: Mark 13:26, Luke 21:27

I would say He referred to himself as "the son of man" to keep His identity a secret until it was the right time to die and let the Jews know He was the messiah. (as he did in Mark 14:61-64) Not only is He the son of God, but He is too the son of Joseph. A man of flesh. This going along with Hammer's answer on March 10 that He is speaking from God's point of view.

Jesus wanted to keep His identity secretive until the right and final moment. Which is why He silenced the demons in Mark 1:34 and others in Matthew 8:3-4, 16:20 & 17:9 so that they would not say He was the son of God and He would be charged with Blasphemy.

I have heard that it is a mystery why Jesus referred to Himself as the Son of Man. And yet it was in fact what Jesus called Himself the most.

I see it like this, Jesus is speaking as God, because He is God. His perspective on everything is from the point of view of God. So what makes Jesus unique foremost, is not that He is a man who is God, but rather that He is God, but also a man. In all the Universe, here is God as "the Son of Man (mankind)". Or - I am God and also a man, not I am a man who is also God.

(Moved from a later "dupicate" question.)

John 12:23 says:

And Jesus answered them, saying, The hour is come, that the Son of man should be glorified.

The expression "son of man" doesn't refer to someone whose father was a male. That would be silly. There would be no need for such an expression. The word "man" means "mankind" or "humanity", not an individual male.

"Son of X" was, and still is today, a common Hebrew idiom. It means that the person or thing referred to has strong characteristics of "X".

For instance, 2Thes 2:3 refers to the anti-christ as "the son of perdition". Similarly "son of man" would be used to emphasize someone's obviously human characteristics.

Jesus is referred to as the "son of man" when it is important to stress his humanness.

Why does Jesus refer to himself as the "Son of Man"? Because he is the Messiah. To understand why he applied this title to himself, we need to go back to the Old Testament prophecy.

After Nebuchadnezzar, king of Babylon, overthrew Jerusalem and took Daniel and his companions captive, Daniel had a prophetic vision about the coming Messiah:

In my vision at night I looked, and there before me was one like a son of man, coming with the clouds of heaven. He approached the Ancient of Days and was led into his presence. He was given authority, glory and sovereign power; all peoples, nations and men of every peoples, nations and men of every language worshipped him. His dominion is an everlasting dominion, that will not pass away, and his kingdom is one that will never be destroyed (Daniel 7:13-14).

This is the first reference in the Bible to the Messiah as the Son of Man, a title that Jesus applied to himself:

He [Jesus] then began to teach them that the Son of Man must suffer many things and be rejected by the elders, chief priests and teachers of the law, and that he must be killed and after three days rise again (Mark 8:31).

The title, ‘Son of Man’ is used 81 times in the Gospels and never used by anyone but Jesus. The New International Version Study Bible makes this comment:

In Daniel 7:13-14 the Son of Man is pictured as a heavenly figure who in the end times is entrusted by God with authority, glory and sovereign power. That Jesus used “Son of Man” as a Messianic title is evident by his use of it (v 31) in juxtaposition to Peter’s use of “Christ” (v 29).

When Jesus used the title He was assigning the Son of Man prophecy to Himself. The Jews of that era would have been intimately familiar with the phrase and to whom it referred. Jesus was proclaiming Himself as the Messiah.

A second meaning of the expression, “son of man” is that when Jesus came to earth he was also a human being:

In the beginning was the Word, and the Word was with God, and the Word was God. He was with God in the beginning. Through him all things were made; without him nothing was made that had been made... The Word became flesh and made his dwelling among us. We have seen his glory, the glory of the One and Only, who came from the Father, full of grace and truth (John 1:1-3; 14).

God called the prophet Ezekiel “son of man” 93 times. God was simply calling Ezekiel a human being. A son of a man is a man. Jesus was both a man and the Messiah. He was also the uncreated Word of God through whom all things were made and as such was also the Son of God, Immanuel – God with us. As the divine being through whom all life was created, he was intimately acquainted with and had full knowledge of humans and why he had to be born of a woman in order to redeem us from our sinful condition.

He was alluding that he would fulfill the prophecy made by the Prophet Daniel in Daniel 7.

13 “In my vision at night I looked, and there before me was one like a son of man,[a] coming with the clouds of heaven. He approached the Ancient of Days and was led into his presence. 14 He was given authority, glory and sovereign power; all nations and peoples of every language worshiped him. His dominion is an everlasting dominion that will not pass away, and his kingdom is one that will never be destroyed.

It was as direct a claim of messiah ship a first century Jew could make. The Jews understood this and is why he was so widely held as a blasphemer by the Jewish people. This is further reiterated by passages like

Mark 2:10

New International Version (©1984) But that you may know that the Son
of Man has authority on earth to forgive sins..." He said to the
paralytic,

The Jews understood that only God had the power to forgive sin. We even have a passage where his disciples admit that he is the messiah.

Luke 9

Peter Declares That Jesus Is the Messiah 18 Once when Jesus was praying in private and his disciples were with him, he asked them, “Who do the crowds say I am?”

19 They replied, “Some say John the Baptist; others say Elijah; and still others, that one of the prophets of long ago has come back to life.”

20 “But what about you?” he asked. “Who do you say I am?”

Peter answered, “God’s Messiah.”