How neutering helps:
- Dogs will live longer, healthier lives
- Eliminates testicular cancer
- Reduces the risk for prostate cancer
- Reduces mammory cancer, cysts, abcesses in the region
- Helps to prevent overpopulation
Behavioral changes:
Neutered dogs are *LESS likely to:
- Roam
- Fight
- Show aggressivness
- Hyperactivity
- Mounting
Note the fact that the behavioral fixes are not necessarily a gaurantee, but neutering may help in many cases.
The reason for behavioral changes in unaltered males is because of the periods where large amounts of testosterone increase in the body.
Testosterone helps hone the animals instincts of survival. Inter-male aggression with other dogs, and threats, establishing territory and behavioral changes in males all attribute to this. They also hone certain characteristics in order to attract females and reproduce.
Hope this helps!
Here are some good resources and articles (including one that dispells the myth of an animal becoming fat because of spay/neuter procedures):