What Sin Is. And Is Not.
You say you're a good person, well have you ever told a lie before? Have you lied even one time? Have you ever stolen something? Jesus said lust is like adultery- have you ever looked at another person with lust? Jesus said anger is like murder- have you ever been unreasonably angry with someone?
Maybe once or twice? Ahh, see, you are a sinner, deserving of God's judgment.
Hold up. That is not what sin is.
So what is sin? Read the rest here: What Sin Is. And Is Not.