Isn't that a little hypocritical, Mr Harkness? She keeps coming back same as you and sure, dying hasn't lost its novelty yet but this is a dream. Anything can happen, sure, but it'll be undone by the time they wake up.
"All the more reason to say it before it's too late. Kind of sucks to hear that on your deathbed when you've already lived your life."
So she'll make more of an effort, with everyone. People should know, right. She gives him a warm, beaming smile, quite happy with this surprise family she's found herself with.