Jim is a somewhat powerful empath, but has been heavily surpressing his abilities since Tarsus IV. However, the mind-meld with Spock Prime on Delta Vega shatters his shields, and he finds himself unable to easily rebuild them. Empaths frequently form spontaneous bonds of varying levels of depth, and part of his trouble comes from a spontaneous bond he forms first with Prime and then with the younger Spock.
The reason his empathy is so powerful is that Kodos had experimented on him, trying to see if he could discover what caused the empathy and make it more effective, or even weaponize it (if anyone's ever seen Firefly, think River in the Academy).
Bonus: Jim had formed a deep, semi-spontaneous bond with another boy on Tarsus. The boy's death scarred him and contributed to his withdrawal
Bonus 2: the empathy isn't just an awareness of what others are feeling. It impacts Jim hard, and he has trouble coping with it. He tends to "reflect" the emotions of those around him. He senses emotions through the filters of the other five senses (since human brains aren't really designed to handle empathy or telepathy); I'd prefer it if most emotions cause a physical sensation, like some kind of pressure against his skin, although it'd also be cool if he 'heard,' 'smelled,' or 'tasted' emotions. Please no "seeing auras"
Bonus 3: his empathy starts to cause a small break with reality as it slowly spirals out of control. He starts acting strangely, hallucinating, etc., and has overwhelming flashbacks to various traumatic experiences in his life
Bonus 4: Winona was actually a good mother and an empath as well. Up to you if she's still alive or if she died on Tarsus.
Up to you where Jim's empathy originally came from. Maybe he's only part human, maybe some humans just happen to be empaths.
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