Android 6.0 Marshmallow comes with plenty of new features, but not all are useful. By default, when you connect a Marshmallow running device to a PC over USB, it’ll connect in “Charge only” mode. To transfer files between PC and the device, you need to pull down the notification bar and select the connection mode to MTP from there to be able to transfer files.
This is an unnecessary extra step, and what’s worse is that you can’t set it to default to MTP when connected to PC via USB. We really hope Google will fix this in upcoming releases of Marshmallow.
Also, until Google fixes this, we hope an Xposed module or a custom ROM like CM13 gives us an option to set MTP as default on Marshmallow.