What motivated Paul to give up his rights and comforts?
Paul was motivated by a desire to see lost people saved. He did not care about material things or comforts; he willingly became as a Jew to the Jews and as under the law to those under the law, so that he might gain them spiritually and awaken their interest.
Les explains that Paul did not care about material things or comforts; all he wanted was lost people saved. This was not always easy, and it can be hard to understand because Paul had been preaching against law, legalism, and temple worship, yet he took a Jewish vow and returned to the temple in Jerusalem to fulfill it. Les says the answer is that Paul did all this in hopes that he might awaken some interest among his fellow Jews. He became as a Jew that he might gain the Jews spiritually, and to those under the law he became as under the law that he might gain them that are under the law, likely referring to the more religious element of Judaism.