Do you konw what your dogs mean?

Try to communicate with your dog (by barking too lol! just kiddin') further. If your dog is barking at a certain direction he might be barking at something at a distant your eyes can't see. If that's the case you might want to let him go there and follow it. If not, maybe he's feeling something painful. But as far as i know if dogs are feeling pain the way they bark is just different. It's just like they bark / cry until they run out of breath.

My first post I have a dog since I was a child. Let me share my experiences; Yes its true, Dogs sometimes barks all day or night without any reasons seeing by human. I have two things in mind; (1) Your dog saw something causing fear. (2) Your dog is not feeling well, why? lets relate it to child 0 to 1 year old. She/He sometimes cried for several hours without knowing the reason behind. Hope it makes sense.