I have reason to be proud of my work for God.

Romans 15:14-17

14 I myself am satisfied about you, my brothers, that you yourselves are full of goodness, filled with all knowledge and able to instruct one another.
15 But on some points I have written to you very boldly by way of reminder, because of the grace given me by God
16 to be a minister of Christ Jesus to the Gentiles in the priestly service of the gospel of God, so that the offering of the Gentiles may be acceptable, sanctified by the Holy Spirit.
17 In Christ Jesus, then, I have reason to be proud of my work for God. – Romans 15:14-17

It is good to feel positive about your achievements. Especially if they are leading people into a closer relationship with Christ. Paul was able to say that the Corinthians were full of goodness. Strange to say that in one sense because Paul has been correcting their behaviours. But he knew God was working in them:

19 and what is the immeasurable greatness of his power toward us who believe, according to the working of his great might
20 that he worked in Christ when he raised him from the dead and seated him at his right hand in the heavenly places,
21 far above all rule and authority and power and dominion, and above every name that is named, not only in this age but also in the one to come. – Ephesians 1:19-21

Paul wrote to them to help remind them of what God had done through Christ for them and what Christ has done for them. We all need reminding. We so easily forget what the Lord has done for us. He has commanded us to love God with all our heart and love our brothers as ourselves. Paul acknowledges that the Corinthian church is filled with knowledge and is able to instruct themselves. We all need to build each other up:

20 But you, beloved, building yourselves up in your most holy faith and praying in the Holy Spirit,
21 keep yourselves in the love of God, waiting for the mercy of our Lord Jesus Christ that leads to eternal life. – Jude 1:20-21

We need to keep ourselves in the love of God. That way our offering of sacrifice will be acceptable to God. We will live as living sacrifices:

I appeal to you therefore, brothers, by the mercies of God, to present your bodies as a living sacrifice, holy and acceptable to God, which is your spiritual worship. – Romans 12:1

Paul was given the grace to be the apostle to the Gentiles. He served the churches that developed in the non-Jewish world. He strove to strengthen the church so that it would not be overcome by the temptations that so easily beset is if we forget what God has done for us. In Jesus Christ, Paul was able to be proud of the work God had prepared for him to do. We to can be proud to be the servants of Christ, not egotistic pride in our own accomplishments but proud to be a part of the wonderful work of redemption God is doing on the earth today