This is... strange, but really cute and heartwarming.
To everyone wondering why he didn't like the fact she might be using her own containers for food: it seems that some restaurants do this for several various reasons, but primarily, to prevent any accidents regarding food poisoning. If you use your own dirty container to take leftovers (often without separating different types of food), then you eat them somewhere and get sick, it's easy to blame restaurant for this and sue them. That's why these restaurants which allow taking leftovers with you, but not in your own containers, use their branded packaging. Also, this is a way to ensure the food is stored properly, and not ruined or mixed up. Also, this could be a way to prevent customers from bringing their own food with them, which is prohibited in many places. Well, try to google it, maybe you will find better explanation than me.