Excel is always asking for credentials on opening a shared document
As a user I don't want to reenter my credentials for a shared Office365-business document/Excel sheet every time on opening them in Word/Excel locally.
(A user shared a word file in it's tenant for me, using my mail-address of my own tenant. Office will ask me every time for my credentials on opening that file, which it should already know. Opening files from my tenant happens without of any bugging requests for credentials...)