Carl could have never imagined befriending someone so closely associated with Negan, the man was poisoned and contaminated anyone around him.. but, he found himself befriending one of Negan's runaway 'wives'. Carl couldn't ignore her presence, and he didn't want to. He had worried about her since he first laid eye on her in The Sanctuary.
Now she was in Alexandria, he found himself trying to give her that comfort he wanted, help her adjust to the community and watch her grow a comfortable look on her face and gain confidence in herself. Carl was hellbent on getting her to enjoy her time in Alexandria after what she had been through, being a possesion by a tyrannical leader instead of a teenager.
He knew what it was like to lose that time of his life, and while their experiences didn't compare completely, they had still lost something in common, and it gave him the ambition to take some of his adolescence back with her.
Carl may not have known her for more than three weeks by now, but he still found they connected. They were similar in a way, through their morality and motivations, and reading comics with her while he ranted about how much a certain superhero 'kind of sucked' definitely helped him bond with her in a way that wasn't brought on by pure survival. He enjoyed acting his age with her, seeing her come out of her shell and feeling himself do it too. The way they began holding conversations were she wasn't afraid to disagree with him. And how she was the one distracting him while he hid with her during The Saviours weekly 50% taking.