Instruction:
Why do you want to keep customers happy?

Response:
You can keep customers happy by putting an extreme focus on them.  Your business should exist to serve the needs of your customer.  You could also call this customer obsession.  By focussing on them and keeping them happy you will reduce customer attrition and churn, and you will have a continuous source of revenue for years to come because these happy customers turn into repeat customers over and over.