Php 1:3-5 I thank my God in all my remembrance of you, 4 always in every prayer of mine for you all making my prayer with joy, 5 because of your partnership in the gospel from the first day until now
Paul planted, others watered but it is God who gave life and growth, in this Paul remembers and gives thanks. Perhaps he is recalling for example how God worked in the conversion of Lydia at Philippi, Acts 16:14 “…The Lord opened her heart to pay attention to what was said by Paul.”, Paul preached (planted), God opened her heart (gave her life).
He pours his gratitude to God for their spiritual gifts, their unity in partnership and their support in spreading the gospel. Paul also personalizes his relations to God calling Him “my God” because of his confidence in the God who saves and keeps. Paul also offers prayer of supplication knowing there was more God was able to do in their lives. His greatest prayer is that their joy would be full and remain.
Thanksgiving is a matter of us rejoicing, and what we have the comfort of, God must have the glory of. Joy gives life to prayer.