Always used to have my dogs neutered as recommended by vets (and by the way, it didn't stop them humping).
however;
"With many retrospective studies to date, there is no clear evidence which indicates neutering male dogs, especially young male dogs, will prevent future health issues.
In fact, the negative health issues far exceed the benefits.
The following information is taken from a meta analysis preformed by Laura J. Sanborn, M.S. in 2007:
Starting with the benefits for neutering male dogs, the procedure eliminates the small risk (<1%) of developing testicular cancer. However, this is a very treatable disease with a high incidence of cure. Neutering may possibly reduce the risk of non-cancerous prostate disorders, perianal fistulas, and diabetes (data inconclusive).
#1: Most pets are neutered prior to their first year of age which disrupts proper hormonal processes. Removing the hormone generating organs of the body so early in age will impact other essential growth processes.
#2: Hormonal disruption in neutered male dogs heighten the risks of other growth centers. Neutering may triple the risk of hypothyroidism.
#3: Early neutering of male dogs increases the risk of developing bone cancer. Osteosarcoma is a common cancer in medium/large and larger breeds with a poor prognosis.
#4: Male dogs who are neutered are more likely to develop other orthopedic diseases. The potential for hip dysplasia and cruciate rupture rises when male dogs have inadequate time to fully hormonally develop and grow healthy bones.
#5: Neutering male dogs increases the risk of cardiac hemangiosarcoma. The likelihood of developing this common cancer in many breeds after neutering rises by a factor of 1.6 and has a very poor prognosis.
#6: Male dogs neutered during their first years have a tripled potential for obesity. Similar to humans, overweight dogs are more susceptible to numerous other health problems.
#7: The originally small risks for prostate and urinary tract cancers increase for neutered male dogs. The risk for urinary tract cancer doubles (<1%) while the risk for prostate cancer quadruples (<0.6%).
#8: The vaccines recommended to pet parents may result in adverse side effects in neutered male dogs. By decreasing the immune stimulation and protection from the testosterone surges during puberty, negative reactions to vaccinations are more likely to occur.
#9: Neutering male dogs may negatively impact their quality of life as they age. Testosterone soaks the brain and provides protection from amyloid deposits, protein deposits that clog brain pathways. The risk of progressive geriatric impairments rises in neutered male dogs.
#10: For the behavioral symptoms pet parents believe will be helped by neutering male dogs, other negative symptoms in behavior may develop. Studies indicate neutered males are susceptible to anxious or fearful behaviors, noise phobias, aggression, and undesirable sexual behaviors.'