WHy should virtual methods be explicitly overridden in C#?

Question

Why should virtual methods be explicitly overridden in C#?

Answer 1

Because it makes code more readable:

class Derived : Base
{
    void Foo();
}

In C++, Foo may or may not be a virtual method, we can't tell by looking at the definition. In C# we know a method is virtual (or isn't) because there is either a virtual or override keyword.

Jason's comment below is a better answer.

(edited for clarity)