even if I am being poured out like a drink offering … I am glad and rejoice: In both Jewish and pagan traditions, people would pour out wine on a sacrifice or at the altar to honor their god. Paul uses this image to say that his whole life is like an offering to God (compare Romans 12:1; 15:16).
Faithful service brings joy because nothing done for God is wasted (see 1 Corinthians 15:58).