In our passage of Scripture, Paul shares how he had learned to secret of being content, independent of his circumstances. This is actually the secret of living in victory over sin, because the opposite of contentment is covetousness, where we are never satisfied. When we do not trust in God's goodness, we easily become discontent, believing the grass is always greener elsewhere. This causes us to move out of God's will, presence and peace to grasp at what we think we are missing, just like Adam and Eve in the Garden. Paul shares his secret in v13 - by allowing the life of Christ within him to satisfy and strengthen his soul, he can be content both in times of lack and in times of abundance.
Oxford Bible Church - Living in the Last Days (audio)