Let's say you decided to rob a bank.
Now we all know you wouldn't really do that right? But for the purpose of this illustration we will pretend you are going to rob a bank.
You walk in, give the teller your note demanding all the money and then flee out the front door. Now because you aren't an experienced bank robber you don't get very far before the cops grab you, cuff you and haul you off to jail.
You stand before the judge in court and plead guilty. I mean, after all, they did catch you red handed. Not to mention all the witnesses at the bank plus the surveillence camera's perfect image of you. So the judge sentences you to 10 years in prison.
Now let's suppose that you look up at the judge and say "But Your Honor, I PROMISE I'll never rob another bank again if you'll just let me go". Would the judge say "OK, you can go free."? Of course not! The judge has a standard of law he must uphold. It's the same way with God. He has a standard He must uphold, too. God is much more just than any human judge. He cannot simply excuse our sin.
We may feel we aren't so bad when we compare ouselves to others but when we compare ourselves to God's standard we all fall short. But God loves us so much He came to take the punishment for our sins upon Himself.