"You mean in my past?" Natasha's brows draw together at the question, like it takes her by surprise. As though she doesn't understand what he's asking. "Never really thought about it."

For a lot of reasons. It's better not to get too attached, isn't it? Whatever you had wouldn't last. Beside that, there's not so much of Natasha's past that's all that enviable.

A few things do come to mind, and possibly ironically some of them do involved Bucky. She hesitates to explain that, though. Not here.

"Could I get a little time to think about it before I answer?"