Without looking it up, explain the idiom "waking up on the wrong side of the bed"

I want to find out how the phrase "waking up on the wrong side of the bed" came to be, but first I want you to give me your best shot at explaining it.
Not asking what it means, but asking why we use it.

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SW-User

If I remember correctly, it came from a superstition that getting up on the left side of the bed or putting your left foot down first gave you bad luck. I don't know who started it. So, people today who get up on the wrong side of the bed are irritable or grumpy....supposedly because of their bad luck. Something along those lines.