Let's just put this out there. Customers are the worst. They pay the bills, but if you're not a people-person, they're frequently demanding, difficult to manage, and unreasonable. Mastering the art of keeping your customers happy requires understanding and work.
That is, you have to start by understanding your customers. Are they indecisive? If so, what does it take to motivate them or overcome their reticence? If they're aggressive, what pushes them over the edge, and how do you manage their aggression?
At the end of the day, you need your customers. And your customers are people. Building strong relationships with them requires the same kind of understanding, patience and effort as any normal relationship. Here's a nifty breakdown by Fundera that will help you start understanding them.