My Dog Keeps On Barking. He's Driving Me Crazy!

I don't know what's wrong with him but for some reason he has been "extra sensitive" today. He keeps on barking even with the slightest sound. Nothing has changed here as far as I'm concerned. Today he started barking at people talking outside again. There are people talking outside everyday and he's used to hearing people outside already and stopped barking at them long ago. Now, he's barking at them again like a new pup in a new house. Every time I go upstairs he starts barking. I can't just let him bark because it's late and the neighbors will get angry. He seems healthy so what could be the problem?