I assume that you already know and do not need to be convinced that:
Your most profitable sales and easiest growth come from existing clients.1
Unhappy customers are 5-20 times more likely to tell others about their bad experience than satisfied customers are to spread good news.2
A simple, high-return method of learning from your happy and unhappy customers, of knowing your customers better while making them more loyal to you is to listen to them. Customers are people and people love being listened to.