Ah, yes. Reminds me of the exact moment I realized my mother had undiagnosed bipolar disorder and possibly a handful of other mental issues.
It's pretty insufferable during the mania. My uncle has it, and it, coupled with him not being the most pleasant person anyway, is pretty tasking. You don't even have to live with him, there will be countless urgent phonecalls and visits, and he'll be just running around town ruining the day for everyone weather he knows them or not.
Well, it's a mental illness, there's no rational reasoning. His arrogance reaches new heights. He has all these great ideas that he has to implement immediately. He has all this energy which would be great if he could channel it correctly, but he can't and he just does everything rushed and half assed. He always needs help for his crazy idea of the day. He becomes delusional, imagining things, convinced of things that haven't happened. He's accusing people of things that they haven't done. He's intimidating people by telling them things that he himself hasn't done (but not even lying, because he probably believes them). Alcohol doesn't help.