Windows 10 has changed the behaviour of the Favourites area in Windows Explorer, now know as Quick Access in Windows 10. This is the area to the right in an Explorer window:
In previous versions of Windows any folder that you dragged to this area would stay there, that behaviour has been changed in Windows 10 to become a sort of dynamic list, but in my experience it doesn't work very well. Here's one solution to the problem, which returns the Quick Access area to predictability.
In an Explorer window click View, then Options:
In the General tab ensure that both options in the Privacy section are unticked:
Predictability and sense should now be restored to the Quick Access area.