For lots of reasons. Anything someone isn't particularly good at doing exhausts them mentally.
Such as, say, math homework. I was godawful at it in high school, and it was always exhausting. Or something I was better at doing, like planning deliveries at my job, can be more mentally stimulating.
Social situations.. gotta do this, should I say this, say it when, oops shouldn't of said that etc, man should i have done this differently yadayada.. it goes on and on. It's so exhausting. =.='
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If Jesus died for my sins, then I should sin as much as possible, so he didn't die for nothing.