I knew a lad who had an English mastiff this dog always seemed very docile, anyway one day we were walking down a footpath when another fella we knew come walking towards us with his Doberman as we drew level the mastiff exploded towards the Doberman and almost put all four of them through the fence, what a shock we all got, I remember the mastiff would stand at the doorway of the house and wouldn't let you out, but was never aggressive think that's where they got the name keeper of the thief