today's other readings are 1 Kings 10: 1-10 and Psalm 37
I cannot say that this gospel passage is one of my favourites, but I do recognize the importance of what Jesus is trying to get us to realize.
Where does sin come from? What may come to mind as answer is Satan, but what Satan actually does is cheer us on when we sin.
No, sin comes from within, from the human heart (see verses 20-21). We choose to sin.
Sin is a willingness to choose evil over God, to turn our backs on God. Such choices have many ramifications, both immediate and long-term.
As the great English author C. S. Lewis once said, “There are only two kinds of people in the end: those who say to God, ‘Thy will be done,’ and those to whom God says, in the end, ‘Thy will be done.’”