Services must be loosely coupled so that they can be developed, deployed, and scaled independently. They of course, need to communicate, but they are independent of each other. They, have well defined interfaces and encapsulate implementation details. But what about data? In the real world and in non-trivial applications (and microservice applications will probably be non-trivial), business transactions must often span multiple services. If you, for example, create a banking application, before you execute the customer's money transfer order, you need to ensure that it will not exceed his account balance. The single database that comes with a monolith application gives us a lot of convenience: atomic transactions, a single place to look for data, and so on.
On the other hand, in the microservices world, different services need to be independent...
