Acht looked at the father and daughter hugged tightly, crying in each other's embrace, and smiled slightly. These two seemed more than capable of standing up again after a hard blow like the death of their loved one. Even Acht himself could never say for sure that he could actually take a hard blow of that caliber and still stand up again.
Losing the most important person in a person's life meant that they will never be able to see them again, hug them again and feel their warmth again. It's a nightmare for any sane human being and Acht was no exception. In fact, it was his worst nightmare since everything he had been doing for the better part of his life was to find happiness along with Dia. That's what he thrived for and that never changed.
So, this scene in front of him was very meaningful for him as he could completely sympathize with them.