The reason for dogs to hate others of their kind may be that they do not want to share, what they think, is their territory with others. This is more of a problem with male dogs who tend to be more dominant. As for humans, he may not feel insecure with them around as he does not consider humans as a real threat. Again he may bark in the presence of another dog because he believes he is responsible for everyone around and wants to act as the protector.
Make clear that you are in charge and responsible, and that he should hold back. If you want to bring another dog home and cannot because your older dog creates a problem, you may try to take him to the second dog in another place where both the dogs have unfamiliar surroundings. Moreover, try to choose a female and a smaller dog such that your older dog does not feel threatened in the second dog's presence. However, if still your dog is behaving in uncontrolled fashion, you need to take him to a behaviorist and seek his advice.
On the other hand, if your dog is a female and otherwise submissive, the problem is perhaps fright. She may have a prior experience where she was hurt by another dog and the memory is still vivid in her mind. In this case she will need your support and must be allowed near nice friendly dogs who will not intimidate her and perhaps after a while she will stop feeling scared.
Make clear that you are in charge and responsible, and that he should hold back. If you want to bring another dog home and cannot because your older dog creates a problem, you may try to take him to the second dog in another place where both the dogs have unfamiliar surroundings. Moreover, try to choose a female and a smaller dog such that your older dog does not feel threatened in the second dog's presence. However, if still your dog is behaving in uncontrolled fashion, you need to take him to a behaviorist and seek his advice.
On the other hand, if your dog is a female and otherwise submissive, the problem is perhaps fright. She may have a prior experience where she was hurt by another dog and the memory is still vivid in her mind. In this case she will need your support and must be allowed near nice friendly dogs who will not intimidate her and perhaps after a while she will stop feeling scared.