Julius, I have had similar thoughts to you about spaying or neutering, though I'm not a dog owner. I doubt the dog "knows" he is sterile. Neutering is supposed to make males less aggresive and less likely to wander, but I suppose you are right to say no guarantees, it is still possible for a neutered dog to be agressive.

Like RP33 says, it's mainly to prevent unwanted puppies that have to be put down.

Ask anyone who works at a humane society what they think of all this.