Here's a simple mechanism for emotional contagion:

1. Mirroring is the behavior in which one person unconsciously imitates the gesture, speech pattern, or attitude of another. -- https://en.wikipedia.org/wiki/Mirroring

2. The facial feedback hypotheisis is that one's facial expression directly effects their emotional experience -- https://en.wikipedia.org/wiki/Facial_feedback_hypothesis

So you subconsciously imitate expressions and gestures from characters in media, even text, which activates corresponding emotional states. The states persist long enough to propagate to people you interact with, like ripples.

It's just that the faster mirroring, feedback and propagation of bits over atoms are generating waves rather than ripples. Beware of tsunamis.