People sin because they have a sinful nature inherited in the womb.
You can see this in Psalm 51.
David says,
"Behold, I was brought forth in iniquity, and in sin did my mother conceive me.
Someone said that the sin of David's mother is in view here, but that is wrong, for the entire context has nothing to do with David's mother.
David is confessing his own personal sin throughout this section of the Psalms.
Have mercy on me, O God
...blot out my transgressions.
Wash me thoroughly from my iniquity
...I know my transgressions.
...Against you...have I sinned.
David is so overwhelmed with the consciousness of his own sin that as he looks back on his life he realizes he was sinful from the beginning. As far back as he can think of himself, he realizes that he had a sinful nature. In fact, when he was born or "brought forth" from his mothers womb, he was
"brought forth in iniquity" (Ps 51:5).
Moreover, even before he was born, he had a sinful disposition: he affirms that at the moment of conception he had a sinful nature, for "in sin did my mother
conceive me" (V.5). Here is a strong statement of the inherent tendency to sin that attaches to our lives from the very beginning.