Multiple realities
(covers information from several alternate timelines)
Emotional transference was a sharing of emotions between two different individuals, causing one to feel the emotions of the other and respond physiologically.
Emotional transference was a side effect of mind melds. In 2258 of the alternate reality, Spock apologized for James T. Kirk's reaction to seeing the destruction of Romulus through Spock's eyes, which included crying, as Kirk lacked Spock’s ability to effectively contain his emotions. (Star Trek)
Emotional transference was also a symptom of Bendii Syndrome (TNG: "Sarek") and Zanthi fever (DS9: "Fascination"), both of which affected telepathic species (Vulcans and Betazoids, respectively).