Responses

Dunno, maybe he's just cold in general, and Judith said she thinks original Ruediger sacrificed himself to save Luca out of duty, not out of real love (even though, the she says Ruediger cares for the child, basically saying the opposite...)
Maybe Luca is just bracing himself to the risk of losing his uncle again?
maybe he's being childish and doesn't wanna share Mommy?
Mine is just a theory, i don't have all the answers.
Also: manga logic of the childish cockblocking trope
Just a theory: what if original-grown-up Luca from the book used the heirloom to turn back time/have a loving family? (So maybe even being responsible for Judith transmigration?)
I remember him being extremely dark and lonely at the end of the book, despite becoming the undisputed lord of the family.
Also young Luca is different/colder and more mature than than he should be, according to Judith. He has skill he shouldn't have learned yet, and seems oddly calm in a new environment (except for when he came back from the heirloom room, but that could be explained).
They say he is more like Ruediger than his father, maybe because his uncle was the one who raised the original Luca?
He also is the main game changer, with how he insisted Judith follow him to the family, calling her mom and being possessive...
Looks likes he has memories of the original storyline, and doesn't want to give up a positive (of a Little clumsy) mother figure now that he finally has one?