Starting with Sparky 7.0 “Orion Belt”, the default configuration of GRUB bootloader does not “see” other operating systems as default.
If the grub config file does not exist, copy it, as root:
sudo cp /etc/default/grub.ucf-dist /etc/dafault/grub
Then edit it, as root, using any text editor, for example nano:
sudo nano /etc/default/grub
adding the following line:
on the end of the file.
Then update grub: