Summary: Jack is one of a kind, much to the Doctor's relief.
The concept of personal space seems to elude Jack. It’s beginning to seem like wherever the Doctor goes, Jack’s there too, standing far too close for the Doctor’s comfort.
It’s not that he doesn’t like his newest companion, the Captain’s a bit full of himself but he’s likeable enough. It’s just that he’s a little too friendly. With everyone. Jack can’t even say ‘Hello’ without it sounding more like an invitation than a greeting.
In all his centuries of travelling with various companions, the Doctor’s never met anyone like Jack, which is probably a good thing. One Jack is plenty.