Changes in Ribbon UI for SharePoint 2013
In SharePoint 2013, selecting a document in a document library does not show the ribbon commands automatically and you need to manually click on the ribbon tabs (Files/ library) to use the commands. In SharePoint 2010, when you select a file checkbox in a document library, the 'Document' tab of the library tools ribbon automatically gets enabled with its commands. This is different in SharePoint 2013 where the Files/Library tab does not get automatically shown on document selection.
However, in SharePoint 2013, we can see that there is a " ... " button appear in the list/library, by clicking it, we can choose to perform the most commonly used actions against the page, list or other components. That means we can perform the actions of our choice within the list/library area in a page. What's more, we also can add custom actions into this " ... " menu to implement more functionalities.
This is an expected behavior in SharePoint 2013. The new UI reflects our “Minimal Download Strategy” in SharePoint 2013 that improves rendering performance when browsing content where large parts of the page do not change, which provides a more fluid navigation experience. You can read more here.
Also refer to the below links which provide more information about the SharePoint 2013 UI experience on document libraries.
# Introduction to libraries in SharePoint 2013 which shows the UI design for SharePoint 2013 document libraries.