While you can focus on the reason he is this way you could also take steps to keep this from wearing you down. There's a book called The Four Agreements by Don Miguel Ruiz. It teaches you to not take things personally therefore you don't react negatively to it. It's hard but once you master it then you can teach your children how to ignore the negative in the world.
I would immediately suspect that there's something at work that makes him miserable and you are just catching the aftermath. Traffic trouble or the new dj on his favorite radio station? There could very well be something at home that is triggering it. Talk to him, if he says he has no idea why then he needs to seek help and you have to tell him to do so for the sake of his family and marriage. Do this as emotionless as possible.
If there's something bothering him and he can identify it then you can help him either accept it and adapt or the two of you can work together to change it.
After 15 years his hormones could be out of whack, Andropause. If he hasn't had a yearly physical then maybe a trip to the doctor is in order.
If your husband is my husband's age or older I would suspect physical problems first then work my way up to emotional or environmental factors.