Why the pop-up about Applications permission on Galaxy S7 appears frequently?

Android 6.0 Marshmallow introduced a new Permissions model with the addition of runtime permissions. Until Android 5.x Lollipop, you had to accept all permissions (such as accessing camera, data network) which an application ask for during the application installation procedure. But from Android 6.0 Marshmallow, the pop-up for each permission appears when the application request to access that permission first time.