no study text says any logic, and even googled. i never found any reason, all info asks us to by heart this rule. one logic i use, debit entry shows what we own, credit entry shows what we give from what we have or liability. in that sense, sales is selling of finished goods, debit shows how much goods we own, sales shows how much we sold what we own so it is a credit entry. this is my logic, not the standard one.