Intellectually, Jack understands regeneration. He knows that the history, the memories and the life are the same. Emotionally, Jack just watched the man he loved die, be consumed by fire, and a stranger take his place.
Intellectually, he knows that a Time Lord who's just regenerated needs support, stability, and reassurance. Emotionally, it's all Jack can do just to hold his ground, and wait.
It's complicated, messy and imperfect. The rigidity and tension, clenched jaw and stiff spine of rage, and barely held in check tears of loss.
It's not the right reaction.
But it's the only one he's got.