Thistle takes things very literally. It isn't just the food being tasty, but being different. Not to mention the satisfaction in the act of raising and preparing it yourself (or with family, friends, etc.). Laius still would have been stuck in the dungeon even if he'd explained that, but instead...
Sweet Jesus, I came into this manga expecting exactly this kind of thing, but it still hurts when it happens. This is hard to continue, but I'm already fully invested.
@Heimdal
It's just to add tension/stakes. We all know how this ends. Someone slips up, she wastes all of them. With friends, I guess. I do feel the frustration though; I prefer watching her annihilate a whole room full of assailants at high speed.
Inb4 Laius f¥cks some sanity into Thistle. Or, more likely, inadvertently causes Thistle to fall in love with him after cooking a decent meal, which Laius will inevitably be oblivious to, as he is with all contenders.