I know what you mean. That exact thing has worried me at points in Doctor Who (because every book discussion we have comes back to DW). Jackie calls Rose on it at the end of season 1, and it's always been this problematic thread for me. Your life is boring and you work in a department store, therefore it's all meaningless and you can run off with a stranger at the drop of a hat? But of course in DW, they all go home eventually, whether they want to or not.

Didn't bug me in Neverwhere, though. Dunno why.