Who would ever choose to be a slave?  In the beginning, one man chose.  Adam threw away the goodness that God had created in him, and smashed the image of God.  In the disobedience of Adam, all mankind was forced to live in slavery, always under the Law and sin and death.
We are conceived and born into that slavery.  We have never had true, spiritual freedom.  We have always been slaves.
But a moment came in history when a second Man was able to make a choice.  He was born of a woman named Mary.  By His virgin birth, Jesus was conceived and delivered without sin.  So the Law could not threaten Him.  He did not need to be a slave.
Yet He decided to be born under the Law, even though He did not have to be.  Christ Jesus voluntarily became a slave, obligated to obey the commandments.  He took the full burden of the Law so that He had to obey perfectly, all the time.

Since in Christ you are no longer slaves of sin, what are you?  You are what Jesus is.  You have been adopted as sons of God.  You have the privileges of sons, including full and open access to speak to God.  You address Him as “Father.”  He really is your Father, in Christ.  As sinful slaves, it would be blasphemy to call God your Father.  But in Christ His Son, you have this privilege.