I don't know that you've ever noticed, given the fact I wear long sleeves 360 days out of 365, but the inside of my forearms are vastly different. The right one is as pale as the rest of me (which is to say, proceed only with proper eye protection when in bright light). The left one, though, is substantially darker. My body seems to have been split down the middle, and one side went one way, and the other went another. In the middle is me, living in the tension.
We're all like that -- if not in visible ways, then in invisible ones. We live in a fallen world, and that fallenness makes sin attractive to us. Why attractive? Because part of us wants it. Our sin nature means we want to sin. It might be murderous rage, lust, greed, deceit, or one of another myriad things, but all of us, if we're honest with ourselves, want something sinful. It's why we can be tempted by these things: they wouldn't be tempting if some part of us didn't want them.
We fight against that temptation daily. We all struggle to stay away from the darkness and remain in the light. John 1 tells us our world is a battle between the two -- but the darkness will never overcome the light (John 1:5). We shouldn't allow it to win in our daily lives, either. That's why Paul says this in Romans 6:1-2: "What shall we say, then? Shall we go on sinning so that grace may increase? By no means! We are those who have died to sin; how can we live in it any longer?"
Each day will feature a battle waged within our very hearts. Each day we face the choice to continue to sin or to resist it and follow Jesus. Which side will we take? Which side will win the battle within?