I think that even when your rational brain KNOWS you are done and is comfortable with that, you still have biological urges for "one more baby."
For me, I know that I am spread too thin with just one child. I should not have any more, because it would probably land me in a mental hospital.
But still, when I read about my pals here at MDC TTC, and getting + test results, buying newborn diapers, picking out names, planning homebirths, I think about being pregnant and laboring and what I would name the baby and all that. But eventually I snap out of that and remember how long it took me to even feel like a human again (it was pretty recently) after dd's birth.
So rationally, I know we're done, but emotionally there's still some pangs.