Alright so I'm running into a common misperception a lot lately... for some reason people think that ALL dogs should get along and be friends or simply work it out until one of them is "in charge" and then life will be peaceful. Sadly this is not the case.
In fact many adult dogs don't get along. They might prefer not to be around puppies, or dislike members of the same sex or feel uncomfortable around larger canines. Whatever their preference some will opt to fight or growl or avoid certain dogs. This is normal and you need to respect your dog's feelings on the matter. This is also why dog parks aren't a great idea for all dogs. It's also a good reason to make sure you allow your dog to meet any potential dogs that you might add to your family beforehand.
If two dogs don't get along and are fighting it is never a good idea to simply "let them work it out". Most fights are all noise and show but if they are allowed to continue it can severely damage any chance of the dogs getting along in the long term. They can also escalate where dogs end up with injuries.