It is certainly not wrong to neuter your dog though some people do think that it is cruel because you are taking away the dogs manhood in a sense. However it can actually help the dog in a number of ways.

In some breeds such as the Saint Bernard, by neutering them you could actually be decreasing their chances of developing cancer and other health problems. The main reason people neuter their dogs however is to stop various behavioral problems. Problems such as aggression, constant excitability and marking their territory can be frustrating and there is a chance that neutering the dog can help. Also many dogs are constantly humping things and that is a sign of sexual frustration and by neutering them it can take those feelings away.

However, overall neautering can help but it does not always work to take away behavioural problems. It is certainly not wrong but it all depends upon your own opinion.

 Neutering your dog is not only good for its health, it can also lead to an additional 1-2 years on his life. Plus, you won't have to worry about your dog finding a "girlfriend".