It's not thought at all. It's the experience of vicarious pain. It might even be the basis for our social tendencies. Best example I know of are neurological studies that suggest that witnessing footage of someone getting stabbed forces you to experience at least a tiny, compartmentalized version of the stabbing, even feeling it in the same spot. It's probably one of the most fundamental parts of the human experience but I personally don't understand it as something I do rationally, but in a good way, if that makes sense?