Oh, DW is normally all about the Doctor for me, too. I grew up watching it (I remember Two but Three was MY Doctor until Ten came along) and while I adored Jo and love Sarah-Jane, Donna's the first companion to come along that I've identified with so strongly and who I've "liked as much" as the Doctor. A lot of that is, I think, due to the fact that I'm closer to her in age than to either of the others, I come from a similar part of London, I share some of her insecurities (although thank God, I get on with my mum much better!) and, as I said over at the rewatch comm, I'm also someone for whom teasing and snarking is a way of showing affection. I was also a fan of CT's before she appeared in the show, which I'm sure is also a factor.