Ah, well, congratulations for adopting an adult dog - even big doggies need love! And it's good that his agression is so seldom. Owners are so often more of a problem than the dogs - you seem to be doing the right thing in redirecting his attention to you. Just keep that up, and prepare a polite "control your own dog, please" speech in your own head for the next time someone's undisciplined pup challenges yours.