When I was at the bank recently making a deposit, I told the teller (she's one of my customers too) that we would no longer be banking with them after 1 more day. This is a bank with which we have done business for the past decade. The very nice teller said, "I understand. It's a business decision." She said this with a smile on her face as she looked me straight in the eyes.
While we gave them advance warning of our departure, most of your customers will not when they decide to leave you. You may never know the reason why. Unless you follow your sales, transaction count, ordering, phone volume, or some other key indicator for your business, how will you know? Would you even consider asking your customer why?
If you have invested in a personal relationship with your customer they will probably go out of their way to do business with you or they may feel compelled to tell you why they have stopped. It's more than conducting a transaction. That's what we have registers or POS systems for. Technology will never be able to replace what we long for, and that's relationship. The face-to-face interaction is so important to the livelihood of most businesses.
So get to know your customer. Know their needs and wants before they do. Otherwise, their decision will be to take their business elsewhere.