People are complicated. Life is complicated. Ecosystems are complicated. And despite our best efforts or intentions, we have a remarkable history of messing things up in unexpected ways.
One simple theory for why this may be is that unintended consequences are likely to happen whenever we try to regulate a complex system by using a simple system. This so often happens in any kind of large-scale or government attempt to control something within a complex system using relatively simple programs, regulations, laws, policies or other actions. Things will happen that we didn’t anticipate.
Examples abound:
Often, as with some of these, the outcome can be the opposite of what you intended, known as the cobra effect. Controlling complex systems is difficult.