Grub2 can remember the last entry you booted from and use this as the default entry to boot from next time.
You may have multiple kernels or operating systems installed in one machine and managed with Grub2 boot loader. Then let Grub2 remember the last boot entry is useful. You don’t need to set a default boot entry any longer.
/etc/default/grub, and change the setting of
gksu gedit /etc/default/grub
2.) Regenerate the configuration file: