The difference between a psychopath and people who just ended up being killers. The other numbers don't enjoy killing, they can relate to people, and they can form healthy bonds. It might not look the same as what other people have, but it's their form of that type of bond. Whereas, my guess is, 003 can't do that. She may have always been that way or it could have happened through some trauma to the head (yes, that's a real thing). She only had any sort of feelings for them 'as numbers'. Not as family or friends or comrades. Her attraction to 002 is not love; it's an obsession. I like that they drew this distinction between the characters.