I'm with LD.
Based on my personal experience, and talking with others, it has 1) to do with low calories (go on a low-calorie, low-fat diet and you'll likely be moody as well), and 2) not enough fat.
I can't stress enough how much difference this last one makes. Most people who eat low-carb aren't getting enough fat, and by extension, aren't getting enough calories. But even if calories are ample, you'll still probably be miserable on a low-carb, high-protein, low-fat diet.
Now there is likely an adaptation period, so that could be the cause. But I've noticed that the more fat I eat, the better I feel and the nicer I am
I can tell you that I have to consciously remind myself to eat more fat--and I'm not at all fat-phobic--like on steak. I even started having a snack of heavy cream (yes, people think I'm gross) in the afternoon, which is both high in fat and high in calories.
My mood has been very good as has my concentration. Dr. Mary Vernon calls this keto-euphoria, which I had never heard of before--but she says she sees it quite often in her patients. (She works with type II diabetics; the ketogenic diet has been quite effective at getting them off their medication.) The key: High ketone levels, which means a high fat intake to go along with very few carbs.
Fish oil may help, too.