(Song to listen to while reading this: Breathe by Jonny Diaz. Or after, if you prefer not to multitask or want to focus on the words of this post and the words of the song.)
One of the main differences in being a son compared to being a servant is that a servant has to work to maintain his position. That is the definition of a servant.
On the other hand, a son doesn't have to 'do' in order to stay as a son. All a son has to do to be a son is just that - be a son. There is nothing a son can do to make him any more or any less a son. He is a son.
We see an example of this in the parable of the prodigal son:
I will set out and go back to my father and say to him: Father, I have sinned against heaven and against you. I am no longer worthy to be called your son; make me like one of your hired servants. So he got up and went to his father.
But while he was still a long way off, his father saw him and was filled with compassion for him; he ran to his son, threw his arms around him and kissed him.
The son said to him, ‘Father, I have sinned against heaven and against you. I am no longer worthy to be called your son.’
But the father said to his servants, ‘Quick! Bring the best robe and put it on him. Put a ring on his finger and sandals on his feet. Bring the fattened calf and kill it. Let’s have a feast and celebrate. For this son of mine was dead and is alive again; he was lost and is found.’ So they began to celebrate. - Luke 15:11-32 (NIV)
In this story, the son wanted to come home and serve his father. He wanted to do. He didn't think he could be a son any more - he thought he had to do in order to be worthy of anything. But he hadn't realized that being a son is not dependent on what he did or didn't do. He was his father's son.
As Jesus and his disciples were on their way, he came to a village where a woman named Martha opened her home to him. She had a sister called Mary, who sat at the Lord’s feet listening to what he said. But Martha was distracted by all the preparations that had to be made. She came to him and asked, “Lord, don’t you care that my sister has left me to do the work by myself? Tell her to help me!”
“Martha, Martha,” the Lord answered, “you are worried and upset about many things, but few things are needed—or indeed only one. Mary has chosen what is better, and it will not be taken away from her.” - Luke 10:38-42 (NIV)
We see here Martha acting as a servant and Mary being a daughter. Mary was not needing to do anything. All that she needed to was to just be and sit at Jesus's feet.
Servants need to do.
Sons and daughters can just be.
(If you haven't listened to the song, go listen now - Breathe by Jonny Diaz)