[Christ Jesus] whom God put forward as a propitiation by his blood, to be received by faith. This was to show God's righteousness, because in his divine forbearance he had passed over former sins. It was to show his righteousness at the present time, so that he might be just and the justifier of the one who has faith in Jesus. — Romans 3:25-26
There are people who hear that God is good, that He is kind and loving and merciful, and they draw from that an erroneous conclusion. On the surface it seems reasonable, and it is certainly pleasant. The problem is that it ignores human reality. The human race is depraved. We can try to gloss over it. We can point to the wonderful works of wonderful, kind, and generous people. We can clean ourselves up and dress ourselves up and do good deeds. Meanwhile we struggle to be honest. We hurt the people closest to us in a myriad of ways — sometimes intentionally, sometimes inadvertently. We think dark thoughts. We crave what we cannot have. We harbor envy and jealousy. Or we simply fail to appreciate what we have been given.
In light of fallen human nature, a genuinely good God cannot simply pat us on the back and say that He understands. God is not like that. He is terrifyingly honest. He does not ignore or gloss over the truth. He has given us all that we have, and He quite rightfully expects us to keep that in mind so that we are able to maintain a proper perspective on life and possessions. Again, we can argue, as I often have, that we are relatively good people. Graded on a curve with Pol Pot, Hitler, and Stalin — heck, with Congress, I don't do too badly. And I can point to lots of people who are better than I am and not necessarily even Christians. But one of the things Paul does in the first three chapters of Romans is take apart brick by brick the argument that humans have any ability to justify themselves against the absolute standard of a holy God.
The perfection of God calls for justice, and justice demands perfection, and we ain't got it. God, in response, offers satisfaction for His own justice. This is the aspect of Christianity that bothers a lot of people — the sacrifice of the perfect, sinless Innocent for the unrighteous. Why could God not simply forgive us? Why could He not just continue forbearing, "passing over former sins"?
The cosmos runs on laws, physical laws, yes, but also moral laws — like the law of the harvest: Whatsoever a man sows, that shall he also reap. Justice is more relentless than gravity. You can overcome gravity for a time by expending enough energy, beating out the acceleration of gravity with a counter acceleration. You can do the same thing with justice by expending moral energy in deception, self or otherwise. But the cosmos demands things balance out. The price for escaping justice must be paid. Against God's plumb line, every human soul would deserve punishment.