I'm pretty sure it's been established that it's a misunderstanding - at least I remember seeing this talk in the comment sections of this series years ago.
In the novel/Luca's first life, Ruediger died when Luca was just a child. Back then, Luca took Ruediger saying that he's responsible for Luca (or something along those lines) that he only cared for Luca due to that sense of responsibility, and not because he actually cared about Luca. He ended up sacrificing himself and dying before they could ever have a chance of resolving the misunderstanding. The incident must've traumatized Luca a lot, and his last moments with Ruediger didn't help.
Even as he grew into adulthood, as he never had a way of properly getting to know Ruediger, of course he'd only have this one idea of him in his head. Most people rarely think about the other point of view, especially when it comes to people they feel they have been wronged by, so that also adds to why he has continued to think of Ruediger as a cold person. (Plus with his recent talk with Judith it's confirmed that Luca being cold to Ruediger isn't because Luca hates him, but quite the opposite. He's also afraid of losing Ruediger, and himself getting hurt again. At least if my memory serves me right lol)
To be honest when Luca said the thing about rudiger that he doesnt know how to love, im starting to doubt him. I like him too. But the doubts ╥﹏╥. I mean, i dont think the things was said without reason right?