People are rebellious, want what they want how they want it and have free will. All sin is rebellion against God and it comes naturally to us as anyone who has ever raised a child can tell you. Children aren't born thinking that lying is wrong they are taught it. Children aren't born knowing that taking something that doesn't belong to them is wrong they need to be taught it and etc etc. We are always tempted and our flesh has an ingrained want to rebel. The good news is that when we are reborn/born again we are filled with the Holy Spirit who gives us the ability to overcome temptation. Sometimes we give in. It happens to the best of us and anyone that claims sinless perfection is a filthy liar. At the same time willful ( consciously choosing to) sin should be an aberration in a Christians life. And if we mess up and do it anyway? If we aren't getting convicted to repent ( turn way from it and not do it again) then we need to spend some serious time on our knees asking God why and for help.