Neurons are not associated one-on-one with a specific function. There is no such thing as a mirror neuron or the mirror system, just as there is no Jennifer Aniston neuron or neurons for a particular emotion or a particular movement. Just as a swallow in a flock is not a “flock swallow”, a neuron which is involved in seeing somebody else perform one of our actions is not a mirror neuron.
Is a thought simply the sum total of the activity of all the neurons in the brain? No, just as a message is not simply the placement of a sequence of letters which follow the rules of grammar. How do those letters stand in relation to that message? How does the activity of neurons stand in relation to a thought? Was Descartes right, or are we our brain?
What causes your depression: serotonin or what you experience? The answer: the question is invalid! Both play a role, and are interconnected. You cannot say that one of these is the cause.