Romans 15:15My children have terrible table manners. If I left them to their own devices they would find it perfectly acceptable to kick, squirm, blow bubbles, show off their chewed up food, throw food, etc. Rather than leave them to their games I choose to correct their poor choices, knowing that their behavior at the table has an impact now (who wants to be a guest at a home where flying food might land on you??) and in the future (business lunches; dates; roommates; social gatherings).
First Paul assures his readers that he has the utmost confidence in their goodness, then he explains why he is willing to be the one to correct their weaknesses. They might have been content on their own to remain in their status quo. They might not have grown up in the right direction; they might not have known what or how to change; they might not have listened to just anyone correcting their actions. Nor would it have been appropriate for just anyone to correct them. But Paul is their spiritual father with authority from the Lord Himself over the Christians he is writing to. He has the calling from God making it right for him to tell them what to keep and what to change. It probably wasn't easy for them to receive his correction, but he has their best in mind. Their actions have an impact on their present (their positive or negative witness to the non-Christians they come into contact with) and their future (eternity). He knows they are called by Christ to become perfect, he is in a position of authority over them, and he loves them enough to say what might be uncomfortable for them to hear.