Maybe he feels that your dog is aggressive or that it’s not well trained? Either way, sounds like he might be wary of it. If it’s a nuisance like barking or lunging though, then you should address it (not sure if that’s the case from your post). In general, no one likes to feel threatened in their own home/yard so you’ll want to address anything that comes across as aggressive or a nuisance to improve the neighbor situation. Also, you could discuss with him the steps you are taking to rectify any bad behaviors so he knows you are trying to improve them (or to figure out if it’s something else altogether). Be firm with training and don’t let your run up to people uninvited or violate quiet hours for your community. And despite your dog being good with some people, keep in mind that not all people have had good experiences with dogs. Also, make sure you have proper insurance for any dog bites/attacks (liability is variable based on state). We can train our dogs for success, but we can’t control what they do 100% of the time so it’s better to be safe than sorry (proper fencing, training, socializing, liability). Do you best to be a good neighbor and pet owner, that’s all you can do! Good luck!