When Matthew saw how Melissa gobble her food without hesitance, he could tell that she's hungry. Despite the fact that her life would be in his hands from now on, still, she ate with gusto.
Matthew anticipated that Melissa wouldn't eat the food he served and will discard it as a trash but to his surprise she ate it without complaint.
Part of him feel relieve that Melissa knows the importance of not wasting food. In spite of the peril he caused her, still she didn't put up a fight when he served her the food, she promptly consume it without questions.
For him its an unacceptable reason if you throw food out of anger. Why would you involve in your animosity the the food offered by your adversary?
Matthew had so many issues when he was growing up. Part of it was how he hated to see someone wasting food in the table. If he ordered something, he made sure that its enough for him or for others to eat.