Why should sickness exist at all? Why can people die when killed? Why do bad things happen to good people? Why isn't life like Heaven for people who haven't done anything wrong?
That's the way it originally was, but sin messes things up. And it's in our very nature. Nobody can say they've done nothing wrong because everyone has at least one concrete sin they've committed, often several.
There's no such thing as a "harmless sin." It has to be black-and-white or else people will try to weasel themselves out of accountability ("I'm only lying, it's not like I'm killing anyone.") Similarly, you can't good-person your way out of being a sinner. Even if you could stop sinning all on your own, if you're not going to repent for what you've done, the guilt is still there.
...Also, as previously stated, you can't fully demonstrate love if you don't show what love is like in hardship.