After encouraging Christians to obey their government leaders and masters in the last chapter, he tells Christian husbands and wives how to treat their spouses. He gives a bit more insight here by pointing out that people will be won for Christ by our selfless character. He admits it is hard to do this, but it will make us like Christ, who suffered even though he was completely righteous.
It takes humility to react positively to negative treatment. The natural response is revenge, lack of cooperation or complaining. The next time you are treated unfairly will be an opportunity to show your godly character or lack of it.