Why does Madison fear “a gradual concentration of the several powers in the same department” of government?

Why does Madison fear “a gradual concentration of the several powers in the same department” of government?
How can government, as Madison claims, be “the greatest of all reflections on human nature”?
How does “the multiplicity of interests” in American society protect the rights of its citizens?
Is Madison’s opinion of the role of self-interest in the conduct of one’s life realistic? Why/why not?
Do you believe the controls against the abuse of power by a single government department have been effective? Why/why not?
Does the federal government equally serve the majority and minority interests today? Why/why not?

https://avalon.law.yale.edu/18th_century/fed51.asp