so that it has become known throughout the whole imperial guard and to all the rest that my imprisonment is for Christ. (ESV)
Paul is in prison. Prison is not a picnic for anyone, least of all Paul. What made it worse was that Paul was not in prison for any sort of legitimate reason. Years prior, before Paul became a Christian, he was putting Christians into prison for no reason. Now the tables were turned. He was now a Christian and imprisoned for his belief in the resurrection of Jesus Christ.
Paul does not take his time in prison as an occasion to work on his legal defense. He does not spend that time writing letters against the oppressive Roman Empire. He is not fixated on how he can change his situation. Rather, he celebrates the fact that God has placed him in a strategic place where the imperial guard of Rome can hear the Gospel!
Few of us will ever experience the level of injustice and tragedy that Paul did. However, we all find ourselves in situations that make us feel imprisoned. What if your current workplace feels like a life sentence? How about viewing it as an opportunity to share Christ with people who would otherwise never hear the Gospel? How about a troubled marriage? Maybe God has you in it to bring redemption to your spouse and your example could heal others? Perhaps school is not a prison, but a mission field?
The most inspiring thing to me about the Apostle Paul is his attitude in the midst of the worst of circumstances. He is able to be content because he knows that God has a plan, and in His graciousness He has allowed Paul to be a part of it. There is no need to complain, but rather a need to allow God to be strong even in our weak moments. Hear the Apostle Paul:
2 Corinthians 12:10
 For the sake of Christ, then, I am content with weaknesses, insults, hardships, persecutions, and calamities. For when I am weak, then I am strong. (ESV)