There is a link between hypothyroidism and aggression in large dogs...if you notice changes in your big guy´s personality, have a simple thyroid test done...it could be as simple as a couple pills a day. It literally saved our Mambo´s life - he was becoming aggressive (very scary) at 3 years of age. I had a few people tell me to put him down as they suspected a brain tumour - I had to try other things first, so I had his vision tested, we got him neutered hoping that would help, and a number of other things but nothing helped...I cried everyday thinking the worst. A friend emailed me as she had heard about hypothyroidism...we had him tested and sure enough his thyroid levels were almost nonexistent! We started him on the meds and within 1 week, the aggression disappeared! He lived the rest of his life on those pills and never bared his teeth at us again...it scares me to think how close we were to having him put down...