Why does Mark Twain lose his job at the grocery store, book store, and drugstore?

Grocery Store:

I had once been a grocery clerk, for one day, but had consumed so much sugar in that time that I was relieved from further duty by the proprietor; said he wanted me outside, so that he could have my custom.

Book Store:

I had been a bookseller’s clerk for awhile, but the customers bothered me so much I could not read with any comfort, and so the proprietor gave me a furlough and forgot to put a limit to it.

Drug Store:

I had clerked in a drug store part of a summer, but my prescriptions were unlucky, and we appeared to sell more stomach pumps than soda water. So I had to go.