I think the age thing is why I identify more with Donna than either Rose or Martha - and in fact, during my S2 rewatch Rose really got on my nerves! Thing is, I thought she was great wth Nine. I saw The Parting of the Ways recently, and they were amazing - they absolutely loved each other and it was beautiful. But with Ten, she turned into a fangirl! Understandable, but annoying!

Back to the point .*g* Moff is spot on in what he says, and I think that's what we're seeing here. It should be a difficult decision to leave your family and wander off to god-knows-where or when, and we'd not really seen that before. It's not that Donna has any doubts about what she wants, it's just not easy.