"Don't sweat the small stuff"

So just a minute or two ago, when I was supposed to be thinking about showering, (the pre-requisite to showering), for some reason, on my mental scroll, the age-old phrase, "don't sweat the small stuff" appeared.
Why?
How the hell should I know.
So it got me thinking, why is that such a commonly used phrase? When you think about it, it really doesn't make any sense at all. How could you possibly SWEAT the small stuff? Like do they mean physically? Is this to be taken literally? (Literal comments are for another date, my friend) And who, might I ask, is "they"? Who wrote this crazy falsity?!

Example No. 1:
Kid: Dad! I got 5/10 on my practice spelling quiz today!
Father: Aw, don't sweat the small stuff kiddo!
Kid: Okay! I'll make a note to stop perspiring misspelled words.

I declare this official Stop Making Up Phrases That Don't Even Make Sense Day where every false, commonly used phrase, which, lets face it, are pretty much only there for awkward silences, are hereby demolished.

And, yes.
This does mean that you may no longer use the "light bulb" classifier in regard to having a good idea unless you have every intention of carrying a light bulb around with you when you have plans of brainstorming.