Customer buys product. A sale is offered a week later. Said customer asks for a reduced price on the already-bought product — essentially, a partial refund (even though they are delighted with the product).
Because we are caring people, we are torn. On the one hand, you sympathize with the customer; you may also not want to alienate them as a future customer. On the other hand, dammit, that product is worth what was paid and a sale is a sale because it is only offered for a defined, limited time.