When Booth talks about the line on that bench at the park, she understands the hidden words far too well. She wishes she didn't, honestly, wishes she was oblivious to this... thing between them. Wishes Angela would stop trying to goad them past the line.
(People who work in high-risk situations, they can't be involved romantically because it leads to things like what happened.)
He needs her to understand that he loves her, has probably loved her for most of their partnership, but can't be in a relationship with her. Because, even now, if something were to happen to her, she knows he would lose it. And she has only has so much objectivity left to refuse (some of) his crazier requests.
She does however worry about his inability to refuse hers, but she can't stop asking, can't stop needing to be near him, no matter the danger.
(Every single day it's with us, there's this line and we can't cross it, you know what I'm saying?)
She knows that one day, they will cross that line together, when they're both ready to accept that the experience, the joy of being more than just partners will be worth the danger it puts them in.