These scenes always rub me the wrong way, since there is such a clear power imbalance between them - he's an employee, they're an employer and the way it's drawn just has this subtle patronizing feel to it that makes it all come across manufactured.
I think most of it is just the mangaka not being skilled enough to make the interaction realistic, but it also just reeks a bit of hierarchies. Like a peasant being welcomed into the noble's house.
I mean look at it another way. They have before them a teenage boy who:
-Is good at housework.
-Makes sure to do his job and do it well.
-Practices respectful professionalism even with a girl from his school.
-Has managed to make a fantastically positive first impression on their youngest, who we were told is typically rather shy.
-Is good at cooking.
-Has given every impression of being a decent, upright young man with no ulterior motives.
All while, I reiterate, being a teenage boy. Think about what most teenage boys are like even at their best, and you'll see how much he stands out. They themselves appear to be decent folk, so it's not surprising they'd want to be appreciative and friendly towards him. Add on the fact that objectively speaking he is a catch for any girl, and the fact that their daughter- who typically has issues with men- appears to like him?
Yeah, I'd be building that port and readying the ship to sail, too.