Why are some believers miserable and unhappy?
A believer's misery stems from old Adam still controlling them. Though God has not removed their free will, they choose to let old Adam reign in their mortal body rather than reckon themselves dead to old Adam and alive to God. They are left with the choice; old Adam does not have to control them.
Les explains that this is the dilemma with a lot of unhappy believers. They are miserable because old Adam is still controlling them, and they are the ones left with the choice. As believers, they still have free will; God has not taken it away. The word 'reckon' means to come to a mental conclusion. The word 'let' implies choice. The admonition is to be led of the Spirit, to live profitable lives, bring honor and glory to the God of glory, by keeping old Adam in subjection. They must not let old Adam continue to reign in their mortal body, but reckon themselves dead indeed unto old Adam and alive unto God through Jesus Christ our Lord. Old Adam does not have to control them.
Let not sin therefore reign in your mortal body, that ye should obey it in the lusts thereof.