It would be giving the writers too much credit to say they're presenting the view that the Doctor is more open to allowing romantic attachments because he's so very alone in the universe now, wouldn't it? It isn't healthy for any of them - Doctor or companions - but it's a way of thinking about it that keeps me from tearing my hair out.
As for Martha's instant attachment and attraction...just don't get me started on that, mmmkay? Ruined a perfectly good character.