Summary: The Doctor is very choosy when it comes to companions.
Over his centuries of travel, the Doctor has become an expert at judging someone’s character within moments of meeting them, and he’s seldom been wrong.
In his opinion, the suitability of someone as a companion should be predicated on their ability to adapt quickly to new situations. He’s met many people on his travels who were pleasant to be around but totally unsuited to life aboard the TARDIS; they would have panicked when confronted with Daleks or Cybermen, most people would, and that’s a very unhelpful reaction in dangerous situations.
His companions have one thing in common; they’re all extraordinary!