Why does Paul keep defending his apostleship throughout his letters?

Les explains that everywhere you read throughout Paul's letters, he is defending his apostleship. It stands to reason because in his early ministry Paul always went first to the Jew, to the synagogue where he would have at least a halfway civil hearing, and when the Jews rejected it, he went out into the Gentile area, the marketplace and whatever. But wherever he went there was that constant slur on the man that he was an imposter, not a true representative of Judaism, and a renegade Jew, so he always had to come back and defend the fact that he is God's man for the hour. Les also notes that in Corinth the church was carnal, not spiritual like Ephesus or Philippi, and beset with problems, including divisions where some said they followed Paul, some Apollos, some Peter, and some even Christ. As Paul addressed that division, he had to defend his apostleship. Les points out that the Corinthians were still co-laborers with the Apostle, which puts us in the same place, that we are co-laborers together to bring to fruition the eternal will of God.