This is just prescriptive (do it because I say so). It doesn't explain anything.
> As other commenters have pointed out, the "double entry" refers to multiple entries within your own ledger, it has nothing to do with someone else's ledger.
I didn't introduce the other guy's ledger, but since you did:
I lost lemonade (which is somehow an addition to my assets). So the "-5" which belongs in the buyer's lemonade - is the negative sign there to indicate that he gained an asset?