That's one thing I tend to like most about a companion - when they're able to bring out different aspects of the Doctor. Not that that should be the only function of a companion, but I tend to focus on their interaction with the Doctor the most of anything.

Rose brought amazing things out of the Doctor - like RTD said one, she humanized him a lot. (For good or bad, that's how I was introduced to the Doctor, and I just don't care for him being all cold and distant.)