We know for a fact that Cain already understands the circumstances of Litia's condition. I think her saying it aloud, about herself, especially with such nonchalance, is what struck an emotional cord with him. He hasn't been around Litia has much as Eugene has, so her sudden composure in the face of death, combined with her cheerful demeanour on most other occasions, has likely left him completely off balance and visibly distraught.