Because if you put it in a sock, you risk it getting stolen or lost in a fire. And it's more of a hassle to pay with it. So the negative interest rate might be

Because if you put it in a sock, you risk it getting stolen or lost in a fire. And it's more of a hassle to pay with it. So the negative interest rate might be worth the safety and convenience of your bank account. As for the reason: it currently costs the bank money to have your money (ECB interest rate on overnight loans is negative), in addition to the costs of operating the bank itself.