Disclaimer: Alas, Bones does not belong to me.
She's going to hurt him someday. She's always known that, and she has a bad feeling that 'someday' is getting closer all the time. The look of barely-veiled panic in his eyes when she mentions being tired of death warns her of that.
But, much though she is willing to sacrifice for him, she is not certain that she can give up her sanity. And that's what she'd be doing, if she didn't take an indefinite break from all of this.
She's not cut out for this, for all of the death and pain, pressure and responsibility. She's not.
And while she's learned so much from her time in the field, she doesn't enjoy this evolutionary process she's been undergoing. Frankly, it's exhausting. She's exhausted.
In the beginning, it was all exhilarating. Now, instead of feeling a bit more dead with each murder victim they find, she feels a bit more alive, and it hurts.
Evolution, she thinks, must be a lot easier when you aren't aware of it.
She ought to just hurt him and get it over with, but she finds herself stalling. It's selfish, really. And even though she knows the best thing she can do for him is to let him go (let him move on), she continues to rationalize procrastination. So she gives herself just one more day (yet again) before she crushes both of their hearts beyond repair.
Today isn't someday, but it's coming soon.