This title, the Son of Man, was Jesus’ favorite designation for Himself. He referred to Himself as Son of Man more than eighty times. Other people never addressed Him that way. He, of course, selected this special designation for Himself with a purpose in mind.
This idiomatic expression was common in the Old Testament. With just one exception, it always referred to a human; thus, Jesus used it in order to emphasize His humanity.
Scripture presents Jesus as a true human being. He was born as a baby, grew up as a child (increasing in wisdom and in stature [Luke 2:40, 52]), and had sisters and brothers (Matt. 13:55-56). He ate (Matt. 9:11), slept (Luke 8:23), was tired (John 4:6), and suffered hunger and thirst (Matt. 4:2, John 19:28). He also experienced sorrow and distress (Matt. 26:37).
To the casual observer, Jesus seemed to be a common man who walked among the people as one of the multitude. Many of His contemporaries did not recognize in Him anything more than a man (John 7:46). People treated Him as one of them; they laughed at Him (Luke 8:53), criticized Him (Matt. 11:19), even mocked and beat Him (Luke 22:63). To them, He was just another human being.
One like the Son of Man came with the clouds of heaven to the Ancient of days, and received eternal dominion, glory, and a kingdom. The Jews identified this Son of Man with the Messiah. So, when Jesus used this title, He was revealing, in a half-concealed way, that He was also the promised Messiah, the incarnate Christ.