Why can't a sinning believer ever be happy?
Because genuine salvation leaves them miserable in sin: they know they are doing wrong, the Lord is dealing with them, and fellow believers may shun them. That conviction comes home. God's discipline has a purpose; it is not mere nastiness, and it concerns believers rather than the lost world.
Les explains that if a person has been genuinely saved, even though he goes out into sin, and if the Lord has not yet taken him out as seen earlier in chapter 5, way down deep he is miserable. He cannot be happy because he knows he is doing wrong, the Lord is dealing with him, and if fellow believers begin to shun him, it really comes home. This is the purpose of it all. It is not to just purposely be nasty, but God has a reason: if you have acquaintances who are believers, ostracize them for a while, but let them know why, because Scripture has admonished us not to have fellowship with those kind of people who claim to be believers. This is about believers, not the world. Verse 12 asks, “for what have I to do to judge them that are without?” Paul has no authority to judge the lost world. Les also goes back to Romans 8, beginning at verse 6, because this is not just one little quirk of Scripture; it is a continuing doctrine throughout Paul's letters on how to behave as a believer—practical, everyday living for believers.