Paul wrote this letter to the Philippians from prison. Despite his situation, this letter uses the language if friendship. There is brotherly love demonstrated between the Philippian church and their first pastor. They keep a God/Christ-centric perspective grounded in the gospel.
Paul commends them for their relationship with Jesus and with him, but he was concerned for their relationships within the church. He encourages them to make more effort to be unified and reminds them that they (including himself) are all servants for Christ. Even Jesus was a servant. Paul also wants to remind us all to be joyful in Christ.