I seriously want to kill my neighbors dogs, chesapeake bay retrievers and they bark at all hours and at every little thing outside. I hate them! 
I know the frustration. From 5am to 8am this morning, the dog across the road was howling. I bet they leave it alone with nothing to do.
They are not small dogs either. They rarely go for any degree of walks, just taken out to do their business. As for your howling dog, it's lonely and wants some company. We had a dog who did that in the unit next door to us when the residents went to a party and left him alone one night. One mournful howl after another, but, they eventually moved out so that problems gone. I should also add that the neighbors with the 2 barkers recently had twin babies and when one wails, so does the other especially since they are apporaching their "terrible 2's" toddler phase. Makes for interesting times trying to get some morning sleep
