It must be said I'm not a fan of "to mate/a mate" conversation. Much like the "orphan/often" discussion in Pirates of Penzance (if you've ever seen it) I think it is a joke that rapidly outstays its welcome.

I think one of the key things Donna did was never let him indulge his tendency to angst. If he had to make a terrible decision or something happened to remind him of Gallifrey etc., she expected him to carry on afterwards, and particularly to continue to behave just as compassionately as he had before.