on OS X, any application that you would normally open as a side effect (opening a document, for example) will close if there is no reason for them to be open an

on OS X, any application that you would normally open as a side effect (opening a document, for example) will close if there is no reason for them to be open anymore. This includes pages, textedit, keynote, preview, etc. They do this because there is no reason for them to remain running if the document is closed. Typically, if you launch the application by itself and it asks you to open a document - these applications will close themselves. On the other hand, applications that don't need to open a document to run, or don't open as a side effect of opening a file, will remain running if the active window is closed. This includes mail, messages, finder. The user benefits from these applications running after the user has closed the active window, since the user might want to launch them again quickly.