Why did Jesus often refer to himself as the Son of Man?

The term "son of man" was an Old Testament poetical synonym for man, such as in Numbers 23:19, "God is not a man, that he should lie, nor a son of man, that he should change his mind," Psalm 144:3, "O LORD, what is man that you care for him, the son of man that you think of him?" or Job 25:5-6, "If even the moon is not bright and the stars are not pure in his eyes, how much less man, who is but a maggot -- a son of man, who is only a worm!" It was most often used by God when addressing the prophet Ezekiel (more than ninety times). Jesus used the title for himself as recorded over eighty times in the four Gospels, but it was never used by his disciples. Some of the early church fathers believed that the use of this title was out of the humility of Christ's human nature, as described in Isaiah 53:2-5: "He grew up before him like a tender shoot, and like a root out of dry ground. He had no beauty or majesty to attract us to him, nothing in his appearance that we should desire him. He was despised and rejected by men, a man of sorrows, and familiar with suffering. Like one from whom men hide their faces he was despised, and we esteemed him not. Surely he took up our infirmities and carried our sorrows, yet we considered him stricken by God, smitten by him, and afflicted. But he was pierced for our transgressions, he was crushed for our iniquities; the punishment that brought us peace was upon him, and by his wounds we are healed." Nonetheless, it was of less controversy than the Son of God or Christ the Messiah, of which Jesus warned his disciples early on in his ministry to keep secret (Matthew 16:20, Mark 8:30, Luke 9:21).

        "Foxes have holes and birds of the air have nests, but the Son of Man has no place to lay his head." (Matthew 8:20)
        "For even the Son of Man did not come to be served, but to serve, and to give his life as a ransom for many." (Mark 10:45)
        "We are going to Jerusalem," he said, "and the Son of Man will be betrayed to the chief priests and teachers of the law. They will condemn him to death and will hand him over to the Gentiles, who will mock him and spit on him, flog him and kill him. Three days later he will rise." (Mark 10:33-34)
        "Blessed are you when men hate you, when they exclude you and insult you and reject your name as evil, because of the Son of Man." (Luke 6:22)

The term "son of man" may have also been used as a synonym for the ideal man. Man, as a created being, was an imperfect image of God (Genesis 1:27), but Jesus, as God's only begotten and born of a woman, was a perfect image of God (John 1:14, 14:9, Hebrews 1:3) and the only man worthy of the throne of God (Matthew 19:28, 25:31, Revelation 3:21). This is in contrast to the "sons of God" referenced in Genesis 6:2-4, which were angels. When God chose Noah to continue on the human race after the flood, he said that Noah was "perfect in his generations" (Genesis 6:9), or without blemish, meaning he was not of the lineage of angels. Thus, when the virgin Mary was found to be with a child of the Holy Spirit, this was an offspring of God and the lineage of man, not of fallen angels, or the sons of God. In the non-canonical Book of Enoch, purportedly written by Noah's great-grandfather, which contains accounts of the fallen angels and their offspring, the Nephilim, Enoch references the future Messiah as "The Son of Man" multiple times. This Son of Man, according to Enoch, was with God before the creation of the Earth, was to be a light to the Gentiles, and would sit on God's throne of judgment over all the world.

        " 'But so that you may know that the Son of Man has authority on earth to forgive sins....' Then he said to the paralytic, 'Get up, take your mat and go home.' And the man got up and went home. When the crowd saw this, they were filled with awe; and they praised God, who had given such authority to men." (Matthew 9:6-8)
        "For as the Father has life in himself, so he has granted the Son to have life in himself. And he has given him authority to judge because he is the Son of Man." (John 5:26-27)
        "Do not work for food that spoils, but for food that endures to eternal life, which the Son of Man will give you. On him God the Father has placed his seal of approval." (John 6:27)
        "No one has ever gone into heaven except the one who came from heaven -- the Son of Man. Just as Moses lifted up the snake in the desert, so the Son of Man must be lifted up, that everyone who believes in him may have eternal life." (John 3:13-15)

The "son of man" was also used as a reference to the coming Messiah in Daniel 7:13-14, Acts 7:56, and Revelation 1:13 and 14:14. Jesus asked his disciples straight forward in Matthew 16:13-17, "Who do people say the Son of Man is?" Peter's reply was, "You are the Christ (Messiah), the Son of the living God," to which Jesus replied, "Blessed are you, Simon son of Jonah, for this was not revealed to you by man, but by my Father in heaven." Matthew 16:27, "For the Son of Man is going to come in his Father's glory with his angels," is reflected in Daniel 7:13-14, "He was given authority, glory and sovereign power; all peoples, nations and men of every language worshiped him." Mark 13:26, "At that time men will see the Son of Man coming in clouds with great power and glory," is reflected in Revelation 14:14, "there before me was a white cloud, and seated on the cloud was one 'like a son of man'." Luke 22:69, "But from now on, the Son of Man will be seated at the right hand of the mighty God," is reflected in Acts 7:56, "I see heaven open and the Son of Man standing at the right hand of God."

        "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 language worshiped 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)
        "I looked, and there before me was a white cloud, and seated on the cloud was one "like a son of man" with a crown of gold on his head and a sharp sickle in his hand. Then another angel came out of the temple and called in a loud voice to him who was sitting on the cloud, 'Take your sickle and reap, because the time to reap has come, for the harvest of the earth is ripe.' So he who was seated on the cloud swung his sickle over the earth, and the earth was harvested." (Revelation 14:14-16)

Therefore, it would seem that the term "Son of Man" was used by Jesus to express himself as the man of sorrows, as the ideal man, and as the promised Messiah. Although many attempt to separate the three and argue just one meaning, each is presented here for your own discernment. It is possible that Jesus was all three, and then some.