The stillness is a response not alien to Shannon, but not really understandable either. He was - is - the favored son of his family, and while that carried heavy expectations, the gravity of which remains somewhat visible, a pervasive sense of entitlement is present too. A war often necessitates the obliteration of boundaries; if he were the perfect soldier feelings and desires and all the other little foibles that make up society wouldn't enter into it at all.
But he's not supposed to be just a soldier, and leading an army requires a different skill set than simply fighting in one. He keeps his hold on her arm for a second longer, ignoring their surroundings, then resumes walking, impeded slightly by his various baggage. "What are you now, twenty? Twenty one?"
Which is to say: she grew up, and that's a little incredible. He should have made sure that happened, but he didn't, and she did fine without him.