I almost titled this "Having a stronger effect on the weak minded," but I'm of the opinion that mindfulness can be learned, even if it doesn't seem to be someone's natural groove. And it just sounds mean.
Angry/negative people can be bad for your brain.
A TON of this relates to what I've studied with Cognitive Behavioral Therapy and NLP. Unhappiness, anger, and anxiety can be contagious. A person can change the quality of emotions in a room/conversation by expressing more positive emotions in their own behavior. If they can accomplish that, their listener(s) respond more positively to arguments or suggestions.
I haven't read about mirror neurons before, but it fits my NLP studies. It also relates to another post I'm working on about creative projects and related social scenes (write, Sassy! WRITE!), will post later.
Furthermore, another possible view on sympathetic magic: "Be the change you want to see in the world." Yay, Ghandi!