I don’t think most people are selfish to the point of it being harmful. I think the problem is that a small number of people are, and those are the people who are in charge of things, where their selfishness can do way more harm.
As others have mentioned, though, a lot of behavior is heavily influenced by the incentive structures people live within. This can apply in very obvious ways: for example, when trying to get from point A to point B, people will use the mode of transportation that makes the most sense for that trip, which is heavily dependent on the infrastructure that exists between those two places, and that’s why the Dutch will bike five miles, the Spanish will catch a train across the whole country, and people in Houston will drive across the street. It can also apply in more subtle ways, though, and that’s where capitalism comes in. To pick one example, companies that are owned by their workers are more stable and better places to work than traditional privately owned or shareholder-owned companies, but it goes far deeper and gets far more complex than that, too.
People are responsive to economic incentives. If the incentives favor doing good things, then good things happen. Otherwise, you get what we have now.