'Get of my back, Grubber.'
'Onwards, my little pony!'
'Just because we are friends doesn't mean I can't zap you.'
'I get off right away, ma'am.'

Tempest for years didn't understand why Grubber wanted to be a guard and why the Princess (Twilight) let him. Everypony would think the little guy after the fall of the Storm King would be happy to go and do whatever he wanted. Not serve under someone else again. Tempest thought maybe he just wanted the shiny life that comes with being a guard and her right hoof. She didn't know what the Princess and Grubber knew. He wanted to stay around Tempest, so he could look out for her.

...But that doesn't mean he didn't enjoy the castle's compfy beds too.

------------------------------NOTE:

No idea why, but I have a soft spot for Grubber, so I had to put him in the AU as Tempest right hoof (and yes, he has his own little armor).
Maybe because I really like to draw him, that could be a reason:P