Since in salvation the soul is regenerated having a sinful nature to a divine nature and you receive the Spirit of God to dwell in you then why do you sin after salvation?
Primarily because sin remains in your old man. Now your old man is mortified (Romans 8:13) but this does not mean that sin in your old man is completely dormant. Sin has a power within itself, Paul writes in about it in Romans 7:8-20. He says that sin uses the Law to lead him to do what he doesn’t want to do. So sin has a power if stimulated.
This is why Paul writes in Romans 6:12-19 that you are a slave to what you submit your members to and he thus warns you to engage in righteous activities. Thus either directly or indirectly if you submit your old man to unrighteous deeds you are stimulating the sin within.
Thus you sin by virtue of awakening sin within your old man by your deeds. This is why you sin. In other words you sin because the sin within your old man sins and not because sin comes from within. Your sin came from within prior to salvation and contaminated your old man but not with a regenerated soul your sin no longer comes from within your soul but from the sin that remains within your old man.
Views from an Evangelist 