This is a blind post so maybe you've dealt with a response like this already.
There's a few different legitimate ways to answer this question. I'll start with just one approach.
- All wrongdoing causes harm.
- Wrongdoing also causes a breach of relationship and the harm caused creates enmity within the relationship.
- The only ways for reconciliation to occur are:
- If the harm is rectified through the wrongdoer's restitution.
- If the harmed party forgives the wrongdoer.
I'll give a concrete example of the above principles. If I steal a TV from you then I harm you to the effect of, say, $500. Knowing that I have harmed you would cause a breach in our relationship and create enmity between us. The only way for me to return to good relationship with you would be for me to restore what I've stolen - let's say $600 for good measure - or for you to forgive my offenses and release me from my obligation to restore.
Please note here that it will cost you at least $500 to forgive me and to reconcile our relationship through forgiveness. An important point to note here is that, while forgiveness is "free" for the offender it is never free for the offended. When forgiveness occurs it's the offended that pays the price.
Now lets turn to our situation with God. In our sin we have harmed God's creation to an effect that is much greater than we can possibly pay back. We cannot restore to God what we owe to him. As a means of reconciling us to himself God offers us forgiveness. But this means that he personally pays for our sins. How does God pay for our sins? The cross of Christ is the answer.
One could go on to ask: "exactly how does Jesus' death on the cross pay for our sins?" This is a good question, but it is another question. For now I'm just trying to answer: "Why can't God simply forgive us? Why is some payment necessary?"
Again, this answer does not exhaust the possible answers to this question. It is just one approach.