St. Paul longed for the return of Jesus Christ because it meant the renewal of his body. It meant the putting off of the temporal tent and the putting on of the eternal, heavenly body. Why? Because he understood that the resurrection and ascension of Jesus meant all the promises of God the Father were going to be fulfilled for those brought into his family through Jesus Christ.
Image: Resurrection of the Dead (lower left section of the Last Judgment), Michelangelo, CC0, via Wikimedia Commons. Image location: https://commons.wikimedia.org/wiki/File:Resurrection_of_the_Dead_(lower_left_section_of_the_Last_Judgment)_MET_MM55713.jpg