I neutered my American Bulldog/Boxer mix at one year old because he was extremely dog aggressive and I naively thought that would help fix the problem. There was absolutely zero change in dog aggression and only with a couple of years of serious training was he able to be trusted around other dogs.
He's 13 years old now. Around 10 years old he began to show the signs of old age, going from an extremely active, powerful and driven cart/rollerblade/bicycle pulling beast (clocked pulling me at 30mph one time)to a lethargic old man that had trouble getting up and going on normal walks. I was pretty bummed, and he didn't seem happy. So, and this is going to be controversial I'm sure, I decided to put him on testosterone therapy. For the past three years I have been giving him a weekly injection of 30mg of testosterone cypionate. The change in his quality of life after two months on the therapy was obvious, he was back to his old self. Coupled with a high quality diet of beef, chicken, organ, etc the last three years of his life have been great. He's not quite like a 2 year old dog, but he can run, get excited, play games and has the drive to be an excellent home guard dog. For 100lb dog that's had an active lifestyle he's far healthier than many of the other dogs I see in his age/weight range.
I'm not advocating this for everyone, but it reaffirmed my belief that neutering a dog is unhealthy and diminishes their quality of life, especially in the later years.