He's a break from our systematic reading. Paul is defending his authority and position. He has been hurt by their quarreling and backbiting. False brethren have come into the church and some have fallen back into their sinful ways. He reminds them that he has visited them twice before and has written them harsh words, in hopes of them repenting. 

He reminds them that he loves them, but if those harsh words helps them, so be it. 

This is a beautiful letter. One which shows deep his love and affection is for the church he planted.