Why the Doctor didn't come back for Amy for 12 years - the whole course of history would change if he had been on time! HIS personal destiny would have changed if he had been on time! If Mels were to succeed in killing him at her younger age, then Rory and Amy would never had ended up having River in the Tardis while traveling, resulting in a regenerating, kidnapped baby trained to destroy the Doctor, but in that case, who would have killed the Doctor? Was the Tardis avoiding paradoxes? Yes.