The foundation of good customer service is the notion that everyone is a customer, at least to some extent.
Customers are obviously. customers. Your boss is clearly a customer, and it clearly makes sense to work hard to give customer satisfaction here!
However, co-workers are customers, where team work is needed. People in other departments are customers when they depend on your work to be able to do their own. And even suppliers are customers, when it comes to making sure that they're paid on time.