Amy Pond and John Smith - better known as The Legs and The Doctor - have been each other’s sworn enemies for as long as they can remember. They’ll do whatever it takes to get rid of each other.
But at the end of the day, they’re two of a kind. No one else is as mad the two, and no one ever will be, and they both know it. They insane sort-of-relationship they’ve got is the only stable and long-lasting one they ever had.