The thing about Bok Dong, Kang Ja realized while looking at this boy that in a flash looked like a man, is that he takes care of her. He always has, or at least he's always tried to. Back when she was Jo Bang Wool, before she really knew he had a crush on her, he handed her his entire wallet to get her out of what he thought was a horrific situation is when it started. He mentions, years later, that it's the first time he'd ever found himself so aware of his hung-nim's payment for him. All the money he didn't care for, when before he just looked forward to his brothers letters, suddenly became so important so he could move her out of the house he thought was so dangerous for her. Kang Ja didn't really know how to feel about it. She's used to fighting uphill battles alone (and when she could finally contact her again, Gong Joo) and here was this boy who tried to fight them for her. He was still a boy to her, though he had grown into himself at twenty two, as he casually shared what feels like such a personal moment she felt a fondness that didn't come as the mother of one of his friend, but as someone who had become friends with the boy independently. She realized that they'd become friends at some point with Go Bok Dong as a semi-permanent fixture around the house, who sent her and Ah Ran letters when he had to do his bit in the army
(a time she looked forward to hearing if he was alright; where she wrote him back every time they got a new letter from him asking him if he was ok, if he was eating enough, if another billion questions that all added up to ease tell me you're safe and sound. He always answered every question to help her peace of mind, but also replied with interest about her well being, he asked her if the house was Ok, about Halmoni, about a billion things that all added up to : are you well, are you happy? ), where he came back to her safe (even if a bit leaner than he left) and happy to see her before he promptly walked past her to check the house wasn't falling apart without him to repair it.
She's had time now to be comfortable in this easy friendship with this boy, who for a second as he came up from under the sink looked at her as the twenty seven year old he was. The moment left in the blink of an eye, and he went back to being the young friend she's known for the past eight years.
Eight years of friendship, where they've taken care of each other, and that second makes her so afraid of ruining it. (Though when she confessed this years after the fact, at 32 and 45 respectively, he laughed and told her he wouldn't have let anything be ruined; she fell in love with him once more, knowing it was true.