In our study of Hebrews chapter five, we will look at the calling of priests and our High Priest which is Jesus Christ (as we saw in chapter four).
"Every high priest is selected from among men and is appointed to represent them in matters related to God, to offer gifts and sacrifices for sins. He is able to deal gently with those who are ignorant and going astray, since he himself is subject to weakness." NIV translation
This passage speaks of the previous Jewish high priests but also applies to Jesus. A high priest was a man that was appointed by God to represent men before God. That is one of the reasons that Jesus had to come to the earth in the form of a man. A part of this representing the people was the sacrifices and Jesus presented Himself to God on our behalf. Another reason that Jesus had to be born a child and grow to be a man was so that He would experience the same struggles and temptations that we face. As we see here, this allows Him to deal gently with us in our failures.
"This is why he has to offer sacrifices for his own sins, as well as for the sins of the people." NIV translation
The Old Testament high priests had to first offer sacrifices and cleanse themselves from their own sins before they could represent the people before God. In the same manner, Jesus had to present the sacrifice for His sin. This is not to say that Jesus actually committed sins (which He did not) but He took our sins upon Himself and in so doing our sins became His.
"No one takes this honor upon himself; he must be called by God, just as Aaron was. So Christ also did not take upon himself the glory of becoming a high priest. But God said to him, 'You are my Son; today I have become your Father.'" NIV translation
Like Aaron, Jesus was chosen by God to represent the people. In order to do so, He had to become a man and so is referred to as the Son of God. Jesus did not presume to do this on His own but was sent by the Father.