As she'd told him at the New Year's Eve party, O'Brien was lucky he didn't have a family of his own to miss. With the way the world was now, she envied that so much. A world filled with zombies would've been easier to deal with if she hadn't lost so many members of her family... especially one member that didn't even have a chance to become part of the family.
Leah didn't expect him to say anything, because there was nothing to say beyond empty promises that things would get better (which they wouldn't). Apart from Evan, and whoever Kori might have told back when they were still angry with each other, Leah hadn't discussed her past with anyone she wasn't related to. A year ago, she would've yelled at anyone who tried asking her personal questions. A year ago, questions about her dead child would've sent her over the edge. She wasn't completely sure that still wouldn't to happen, but for some reason she trusted O'Brien not to abuse this knowledge she'd given to him. He wouldn't hurt her like that.
"I know," she nodded, lifting her hand to cover his and letting her eyes slide closed. "I know. What happened tonight wasn't even about me at all. It was about you getting that asshole to stop badgering you and threatening people." Exhaling, Leah opened her eyes again. "It's just... It's taken me more than four years to let someone get this close again... and I didn't want to lose that. I didn't mean to make this all about me, when you've got family to look out for now. I panicked."
Her hand rested on his shoulder, fingers fiddling distractedly with the collar of his coat. "Guess I should've told you the biggest thing first, instead of working up from little things." Now that she'd told him the most important thing about her, it felt like a great weight had been lifted off her shoulders. Like she could tell him just about anything now.
"I don't like the thought of having to let you go." That could've been interpreted in a lot of ways, the most important meaning resting in the fact that she still wanted him to have his chance. Now that he knew.