It wasn't all that hard to translate what little Din was saying into what he really meant -- a larger picture with layered connotation. Grogu had been more than a child he'd been watching, he'd become his son, part of his people and in giving him up to Luke, it had been denying a claim his heart had made.
It was cheating a little, perhaps, that Luke had seen this in the other version that had been here already. It didn't move him less though, because Luke understood just how important it was to have a family, especially one that was found. He missed his sister and Han often. "I understand," Luke said earnestly, although he couldn't really think of anything else to say. Some other version of hims hadn't been as willing to bridge gaps, so far as he could tell.
Still. When Din suggested he wanted to know him, Luke's smile lit up until it was sunny and dimpled, not as much as a direct contrast to the all-black he was wearing as he might have thought, really. "Then you will. I'd like that, too."