Paul is writing to another church he founded in Philippi. He wants to encourage them to continue to grow in their faith. Paul is currently in prison, but he is not complaining. He is happy he can suffer for the sake of his ministry. He feels suffering for Jesus allows others to see that He is more important to him than anything.
If you were to be imprisoned for Jesus, would it make you happy? Paul has given up everything for Jesus and is happy to have done it. It takes a steady focus on Jesus to live life this way. Paul is a great example to follow!