John Wesley on Romans 15:20

Not where Christ had been named - These places he generally declined, though not altogether, having an holy ambition (so the Greek word means) to make the first proclamation of the gospel in places where it was quite unheard of, in spite of all the difficulty and dangers that attended it. Lest I should only build upon another man's foundation - The providence of God seemed in a special manner, generally, to prevent this, though not entirely, lest the enemies of the apostle, who sought every occasion to set light by him, should have had room to say that he was behind other apostles, not being sufficient for planting of churches himself, but only for preaching where others had been already; or that he declined the more difficult part of the ministry