Peter spent chapter two warning his Christian readers about false teachers and about how their bad thinking/theology has led them into bad behaving/acting. This is why Paul says to watch both your doctrine and your life, the two go together.
When we abandon the authority of the Word of God we soon find ourselves in all kinds of immorality, sexual immorality, abortion, divorce, and justification of all kinds of practices God has clearly warned us to steer clear of (see Ten Commandments).
Now in chapter three Peter speaks more directly to his Christian readers. This is a pastor writing from a pastor’s heart, like a loving parent to his children. This is evident from the term of endearment he starts with.