The way I see it, it is the seed of the Father that empowers us to no longer want to sin. 1 John 3:9. Jesus was conceived from it, thus He did not sin, and the baptism of the Holy Spirit is how we receive the seed of the Father. Those of us who are actually born again have a new nature that is the opposite of the old carnal nature, and enables us to partake of the divine nature of God. 2 Peter 1:2-4; Romans 8:9
The problem I see in your post is that you seem to be taking
choice out of the discussion. We sin because we
choose to. For some reason, in heaven we will choose
not to. Barney is asking why. Why will we choose not to in heaven? Same reason I believe we choose not to now. We have a progressively clearer understanding of who God is. We realise that we don't
need to sin. He will look after us. Our love for Him grows exponentially as we better appreciate His love for us. It is His love for us, best revealed by His giving of Himself a sacrifice for sin, that motivates our love for Him, and the more we love, the less we sin. Thus love becomes the fulfilling of the law.
The question then becomes, do we want to cease from sin now, and how much? They're is nothing in scripture that suggests our characters are changed at the time of the Resurrection. Our bodies yes, but our thoughts and intents of the heart? They must be changed now... And it's God who has graciously provided all the motivation, the means, and the power to do so. The gospel is a thing of power.
KJV Ephesians 1
17 That the God of our Lord Jesus Christ, the Father of glory, may give unto you the spirit of wisdom and revelation in the knowledge of him:
18 The eyes of your understanding being enlightened; that ye may know what is the hope of his calling, and what the riches of the glory of his inheritance in the saints,
19 And what is the exceeding greatness of his power to us-ward who believe, according to the working of his mighty power,
20 Which he wrought in Christ, when he raised him from the dead, and set him at his own right hand in the heavenly places,
It is creative power in the minds and hearts of all who believe, and that power comes to us through revelation of the nature and character of the God who while we were yet sinners, died for us.