It can be painful and disheartening to lose a customer, and it's also an accepted part of running a business. If you really want to improve your offering, and make a significant change to your value proposition, you need to obsess over your ex-customers.
Many companies fear talking to ex-customers, expecting them to be angry or bitter as a result of poor service or product performance. But in amongst the disaffected will be those who've discovered something better, or decided the problem wasn't really worth the cost/effort you were imposing.
If you can quickly identify a shift in attitude or value away from your offering, you can better respond; and you'll be better able to find new ways to deliver value.