Mirror and Circumstances

What is meant by,
"The sight of the environment gives rise to the inner state?"
Suppose we are watching a movie
and our mood follows the scenario of the story,
sometimes happy, sometimes sad.
Although the story has nothing to do with us,
we unwittingly go along with the scenario,
which gives rise to lots of non-liberated,
disturbing, and senselsess tensions.
If our mind and heart can be like a mirror
that is not affected by "circumstantial conditions,"
our life will be peaceful and serene.
So, what are we learning from the Buddha Dharma?
We are learning about a mind and heart of serenity.
What is a mind and heart of serenity?
It is like a mirror.
The mirror reflects everything,
but itself does not have this or that kind of feelings or sensations.
The mirror is simply a mirror.
The image of the circumstances is simply
the image of the circumstances.