From Radio Times
Doctor Who head writer Steven Moffat says the series tells the story of the Doctor's companion, rather than the Time Lord himself, and that the thought of the Doctor travelling alone is "depressing" and "unhealthy".
“The story of Doctor Who is always the story of the companion, it’s always their story," said Moffat. "It was Rose Tyler’s story, it’s Amy Pond’s story - the story of the time they knew the Doctor and how that began; how it developed and how it ended."
Current companions Amy Pond and Rory Williams - aka Karen Gillan and Arthur Darvill - are set to leave the show partway through the upcoming series, with Jenna-Louise Coleman starting a new chapter when she makes her first appearance in the Christmas special.
“The story begins again, not so much with the new Doctor, but with the new companion," Moffat told BBC America. "It is their story. The Doctor’s the hero, but they’re the main character.”
“I thought about the Doctor travelling on his own and it always faintly depresses me,” added Moffat. “I’m not sure what he does on his own but I don’t think it would be healthy. He’s far too old and he’s seen too much.”