[JeanMcaleer]

It's completely normal for dogs to bark at other dogs. They communicate this way, and it can mean various things. As you mentioned, some dogs bark as a way to say "hello" or to alert their owners to something unusual.
The frequency, intensity, and context of your dog's barks can provide insights into their motivation. If you want to understand your dog's barking better, you can read more about it in the article about Why do dogs love tennis balls. It might help shed some light on this behavior.
Remember that training and socialization can also play a significant role in reducing excessive barking.