I have a theory that whenever they see a cute girl on an adventure, The Doctor and Clara go back to the TARDIS and fangirl about her.
"Omg Doctor why did you not let Journey come with"
"Because she be being a soldier"
"But she be being a hot soldier with a cute pixie cut"
"But-"
"She's cute we should invite her for a threesome"
"Just this once"
But it wasn't just that once
