More personal canon, from something that's crossed my Tumblr dash frequently (
here is my reblog). I think Jack runs around behind the Doctor and takes care of the companions. The Doctor loves them all, but he's been around for so long and has had so many that when they leave him he doesn't always seem to go back to them, and Jack is there making sure they're taken care of and happy.
This has also got me pondering this: if Jack's taking care of everybody else, who's taking care of Jack?