Donna Noble, force of nature; she’s unstoppable when she’s like this, a bright flame of righteous fury, as fiery as her red hair and with a temper to match, He can’t help loving her, but just as friends, not in any romantic way. She’d bite his head off if the thought ever so much as crossed his mind.
What an incredible woman she is though, so completely human, full of compassion and spirit; she gives him hope for humanity, that those primitive apes have the potential to grow into something worth saving. Some day they’ll take their rightful place in the universe; the Doctor knows this because they already have, and people like Donna are the reason and the impetus that made, or will make, it happen.
As she turns the full force of her indignation on those who foolishly tried to cross her, the Doctor prudently steps back; he doesn’t want to get in the firing line. She’s really scary when she gets like this and not for the first time, he thanks all the fates that she’s on his side. He wouldn’t want to get on the wrong side of her; she’d be a formidable enemy.
He’d much rather have her as a friend.