Why is the mirror an old friend? Because it always sees us and is always with us.
Думаю так. НО если этот вопрос задан по контексту, то вероятность правильности мала.

Because it shows our reflection and if we didn't have the mirrow, we couldn't see how we look like. And also, figuratively, we can call with the word "mirror" our real friends. They are the ones who help us see ourselves from the different angle. They help us to understand our behaviour. So the first one helps us to see us from outside and the second one - from inside.