OK, let's say that you haven't had a bath in two weeks. You'd be all gross, wouldn't you? You'd be sweaty, dirty, and let's face it: you'd be one ripe- smelling kid. Even if you decided to be "good" -- you didn't do a few rounds on the basketball court, and you only went outside to get the mail for your mom -- you'd still be all "yuck".
Before we ask God to be the boss of our lives, we're just like that -- but on the inside. When we become a Christian, it's just like we took a shower! We're clean now, and we feel great!