What makes me irritable is, ironically, usually aspie things, such as being interrupted while concentrating on special interests, and changes in my daily routine. For this reason, most people think I'm just naturally an unhappy and mean person, however this is far from true. The only other thing that makes me irritable is stress, but that's a whole different story. As far as I know, the best way to eliminate irritability is to eliminate what causes it. In my situation, that's hard to do because the things that make me irritable are the very things that are out of my control.
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Remember, all atrocities begin in a sensible place.