Instead of going to Rome as just a preacher; Paul ended up in Rome as a prisoner, and his journey to Rome was no walk in the park. (See Acts 21-28) Paul could have written a best-selling autobiography of his epic journey from Jerusalem to Rome, but instead summarises the whole story in Philippians 1:12 with just the words, "...the things which happened unto me..."
Most of us would look at this and conclude the whole ordeal a complete and utter failure. This was not necessarily how Paul planned his arrival in Rome, but Paul's joy was not rooted in his circumstances. It was rooted in a person, the person Jesus Christ.
Jesus was Paul's whole reason for living. (See 2 Corinthians 5:14–15) To Paul, Jesus was the source and the end of it all, and in the midst of the worst possible circumstances, Paul set His eyes squarely upon the one who is both our means and our end.
Roy Hession makes this observation in We Would See Jesus:
We have been availing ourselves of Jesus and His blood as the way, but to ends other than Himself.[i]
To have joy in the rain, we must see Jesus as the umbrella in the rain, and the warm and dry destination through the rain. When life does not go your way pick up the umbrella and set your eyes squarely on your destination. Look to Jesus.
[i] Hession, Roy. (1958). We Would See Jesus. pg. 122. Fort Washington, PA USA: CLC Publications.