What motivated Paul to preach to the Jews first in each city?
Paul went first to the synagogue in each city because Jews were present in every city of importance, and, as a Jew steeped in Judaism, he had a love for his kinsmen according to the flesh. He expounded the Old Testament to them, since no New Testament writings yet existed.
Les explains that by the time of Paul's missionary travels, Jews had been dispersed into every city of the Roman Empire. Therefore, wherever Paul went, he went first to the Jew, to the synagogue, because they were in every city of any import. Being a Jew himself, steeped in Judaism, and having a love for his kinsmen according to the flesh, he would expound to them out of the Old Testament. There was no New Testament yet; the Gospels had not been written until after Paul wrote his letters, so he had to speak the Word verbally, because nothing was written until he began his letters.