All In the Same Boat
In praying to God on behalf of the Jewish people, Moses asks how it can be that one man (Korach) transgresses and God becomes upset with the entire nation. Does one man really affect the whole nation?
The answer in Jewish thinking is, of course, yes. From God's point of view it's easy to see - we are responsible for one another, and if one person is doing something wrong then we all share the blame.
This can also be understood from a practical perspective. The Sages give a lovely analogy. They say it is similar to a person who is sitting in a boat full of people and drilling a hole under his own seat. The people scream at him to stop, but he cannot understand why they are upset - after all, he's only drilling under his own seat, not theirs!
It's easy to see that people chopping down rainforests in Brazil ultimately affect people in Australia. And how if China is filling the atmosphere with greenhouse gasses, we may get ourselves a few more deserts. However, the boat analogy works on a spiritual level, too.
We all live together on a boat and that boat is our society. If it sinks, we all sink. If it gets somewhere, we all get there. If a person is immoral in his own home with no one watching, he is still drilling a hole under his seat. Because by lowering his own standards of morality, he affects those he interacts with. Like it or not, we respond to each other. Good and decent people lift us, and lowly people drag us down. People are pulled after their environment and each of us is part of each other's environment. We affect each other by who we are and how we live.
One man's transgression does affect the whole nation - albeit in a small way. But lots of small decisions can create a spiritual "butterfly effect." Our decisions and our actions count - not only for our own lives and for those immediately around us, but ultimately, the effects are felt by our whole society.