Questions: Romans 15: 17-21 and Corinthians 10: 14-17

Paul’s not boasting in another man’s rule.

Romans 15:17-21
17 I have therefore cause that I may glory through Jesus Christ in those things which pertain to God. 
18 For I will not dare to speak of any of those things which Christ hath not wrought by me, to make the Gentiles obedient by word and deed, 
19 through mighty signs and wonders by the power of the Spirit of God; so that from Jerusalem and round about into Illyricum, I have fully preached the Gospel of Christ. 
20 Yea, so have I striven to preach the Gospel not where Christ was named, lest I should build upon another man’s foundation; 
21 but as it is written: "To whom He was not spoken of, they shall see; and they that have not heard shall understand."

2 Corinthians 10:14-17
14 For we stretch not ourselves beyond our measure, as though we reached not unto you. For we have come as far as to you also in preaching the Gospel of Christ, 
15 not boasting of things beyond our own measure, that is, of other men’s labors; but having hope that, when your faith has increased, we shall be magnified in you according to our rule abundantly, 
16 to preach the Gospel in the regions beyond you, and not to boast in another man’s rule, which he made ready for our hand. 17 But "he that glorieth, let him glory in the Lord."

In Acts 22:17,18 and 21 we can read what commission the Lord gave to Paul: 
17 "And it came to pass that when I had come again to Jerusalem, even while I prayed in the temple, I was in a trance 18 and saw Him saying unto me, ‘Make haste and get thee quickly out of Jerusalem, for they will not receive thy testimony concerning Me.’ 
21 And He said unto me, ‘Depart, for I will send thee far hence, unto the Gentiles.’"

So Paul had no commission for the Jewish land and Jerusalem, but for heathen countries, far away. He accordingly worked and preached the gospel in towns and regions where the gospel had never been heard. That will be no difficulty for any reader of Gods Word. 
Another question is, why he wrote in terms that seem to indicate, that he used to boast of the work he did. 
Let it be clear, that he was not used to boast. 
In 2 Corinthians 11:1 he wrote: 
Would to God you could bear with me a little in my folly.
So he thought it to be folly to boast. Likewise in verse 16-19
I say again, let no man think me a fool. But if otherwise, yet as a fool receive me, that I may boast of myself a little. 
17 (That which I now speak, I speak it not from the Lord, but as it were foolishly, in this confidence as boasting. 
18 Since many glory in the flesh, I will glory also.) 
19 For ye suffer fools gladly, seeing ye yourselves are wise!

Still, why has he written those passages in Romans and in 2 Corinthians? 
In Romans 12-15:16 he had admonished the believers in a way that made him add the soothing verse 14
And I myself also am persuaded about you, my brethren, that ye also are full of goodness, filled with all knowledge, able also to admonish one another.

And, realizing that for the believers in Rome he was not the apostle through whom they had heard the gospel, he added again:
Nevertheless, brethren, I have written even more boldly unto you in some measure to remind you, because of the grace that is given to me from God, 
16 that I should be the minister of Jesus Christ to the Gentiles, ministering the Gospel of God, that the offering up of the Gentiles might be acceptable, being sanctified by the Holy Spirit. 

Would the believers in Rome fully accept his authority, important not for him, but for the acceptance of the truth he preached? In view of those considerations the apostle wrote the words in verses 17-21. 
Moreover it made clear why he, intending to visit them, had postponed it. He had still seen regions before him, where nobody had preached the gospel and he felt it to be his first responsibility to fulfil his commission there. 
How grateful we ought to be, that he therefore has written this epistle. Otherwise we would not have had this marvellous exposition of the way of salvation. 

The reason to write the passage in 2 Corinthians is different. 
Paul had adversaries, particularly under Jewish teachers, who tried to bring the heathen believers under the Jewish law. We find the traces of their endeavours in various epistles. Paul resisted their damaging work. His opponents cast suspicions on him and undermined his reputation and authority. We see it very clearly in 2 Corinthians 11:5 and in verse 12, 13
But what I do, that I will continue to do, that I may cut off occasion from those who desire occasion, that wherein they glory they may be found even as we. 
13 For such are false apostles, deceitful workers, transforming themselves into the apostles of Christ.

Likewise in verses 21-33
I speak as reproached, as though we had been weak. However it be, whereinsoever anyone is bold (I speak foolishly), I am bold also. 
22 Are they Hebrews? So am I. Are they Israelites? So am I. Are they the seed of Abraham? So am I. 
23 Are they ministers of Christ? (I speak as a fool) I am more—in labors more abundant, in stripes beyond measure, in prisons more frequent, in deaths often. 

Against his will the apostle had been forced to stress his authority and apostleship. Not because he liked that for himself, but because he saw the danger that his teaching would be rejected by some. 
In chapter 13:3-5 he even asked the readers to realize, that they, being believers, had received the saving message by him, which was proof that the Lord had sent him: 
since ye seek a proof of Christ speaking in me, which toward you is not weak, but is mighty in you. 
4 For though He was crucified through weakness, yet He liveth by the power of God. For we also are weak in Him, but we shall live with Him by the power of God toward you. 
Examine yourselves whether ye be in the faith. Test your own selves. Know ye not yourselves how Jesus Christ is in you, unless ye be reprobates? 

The second part of verse 3: “which toward you is not….. till verse 5 is a parenthesis. The meaning is: When you seek a proof of Christ speaking in me, that proof you are yourself, for you have heard the gospel and believed by me. 
This will be enough to make clear, that Paul did not like to boast, but was forced to do it in order his important teaching would not be thrown away by some to their great spiritual loss.

J. Ph. Buddingh