Why does separation of powers seek to prevent people from being judges in their own causes?

The principle that individuals or groups should not be "judges in their own cause" is a fundamental rationale for the separation of powers. This doctrine seeks to prevent the concentration of authority by ensuring that those who formulate the laws are distinct from those responsible for their interpretation, application, and enforcement.1

By dividing these functions among separate and independent branches, the system creates a structure where lawmakers are subject to the same laws they create. This provides a necessary incentive for them to avoid self-interested legislation and instead frame laws in general terms that apply equally to all citizens, including the lawmakers themselves.1

When the judiciary and executive branches operate independently, they are similarly bound by these laws, which encourages impartial decision-making. Ultimately, this separation aims to limit arbitrary government excesses and safeguard individual liberty by ensuring that no single entity holds the power to both create the rules and serve as the final arbiter of their application.21