Every time that you want to link to your Dropbox account, Dropbox will ask you for your credentials (username and password).
I know that this is very boring but unfortunately it's out of my control!
As workaround, you can install the official Dropbox app. In this case, every time you want
to link to your Dropbox account, the Dropbox app will automatically start and, if already linked, it will use the
linked account and you will have just to accept or deny the credentials by pressing a single button.