“I once heard a theory about the first relationship that occurs after a big relationship ends. It’s called the 90/10 rule. The theory goes: whatever the crucial 10 per cent is that was missing from your partner who was otherwise totally right for you is the thing you look for in the following person. That missing 10 per cent becomes such a fixation that, when you do find someone who has it, you ignore the fact they don’t have the other 90 per cent that the previous partner had.”