Back to the original question/subject: "reasons for altering":
I will admit that having a giant breed with his testicles intact does invite some negative behavior from a handful of dogs (and owners). I've had a guy (with a neutered black lab that went ballistic upon seeing- and smelling, I assume- Mateo), literally have to pick his lunging dog up and slam him into a construction site fencing as we passed by, cursing and screaming at me that, "this wouldn't happen if you got your dog neutered!!!".
Well. Okay then-- I will run right over and schedule a surgery just to make your dog more comfortable...
And there is another person who has 2 dogs, one of which wants to kill mine...just because (she says) "he's not neutered." Well, she wasn't kidding. A few months ago, she lost control of the leash as he was lunging towards Mateo and attacked. It took 2 men to subdue this dog, with one pressing his knee on his head. Meanwhile, Mateo stood calmly by my side, even after having to engage in a fight he never asked for. The only thing she had to say to me was, "If you got him neutered my dog would leave him alone!"
Again, it's not up to me to fix your dog's training issues, lady....
I'm just illustrating why the above examples might be a reason to neuter...it may well help dogs interact on a more even playing field, I don't know. I do have a friend here in the city who recently got her dog neutered for just this reason. Personally, it's not enough to persuade me to alter my dog. If, in addition, he had some major behavioral or medical issues on top of everything else, than of course it's a whole diferent story...