During the latest mission, the last thing Bruce expected was to find a small baby. Your parents died and that's why you were left alone, and until information about the rest of your family was found, Bruce took you in.
He was very scared every time you started crying; he was used to his children being independent and able to stand up for themselves, to be older than few months old, but to realize that he is responsible for a small person who can't do anything on their own... It scares him.
Alfred helped him at first, telling him what to do in certain situations, but Bruce still wasn't sure that he was looking after you well. "You're so cute." Bruce smiled, looking at you in your new onesie, happily kicking the air with your legs. "You're fed, clean... God." He sighs, realizing that he needs to go on patrol, and he will have to leave you to Alfred or one of his children. What if something goes wrong? What if you cry because you miss him? To stop himself from spiralling into full-blown breakdown, he just lifts you up in his arms. You're so small. Innocent.