Java では静的メソッドが非静的フィールドにアクセスできないのはなぜですか?

public class NonStaticFieldReference {

    private float r; // Non-static field

    public static void main(String[] args) {
        System.out.println(r); // Error!
        c_area(); // Error!
    }

    private static void c_area() {
        // Cannot reference non-static field r from static method
    }
}

public class NonStaticFieldReference {

    private float r; // Non-static field

    public static void main(String[] args) {
        NonStaticFieldReference instance = new NonStaticFieldReference();
        instance.r = 5;
        instance.c_area();
    }

    private void c_area() {
        // Access non-static field r from non-static method
    }
}