IF the falshforward is reality

I would say Jack has a SEVERE case of PTSD trauma worse than others
He was tortured
forced to do an operation
mercy killed Libby

saw death of many

combat medics often have undiagnosed PTSD
and Jack may be self medicating for many reasons including guilt powerless to help others...

"Maybe the Kate and everyone else adjusted better than Jack."

MAYBE cuz he was the Doc who failed the good sheperd who LOST his flock??????