In the very first chapter of the very first book, we observe the following exchange between Professor McGonagall and Professor Dumbledore:
"You think it -- wise -- to trust Hagrid with something as important as this?"
"I would trust Hagrid with my life," said Dumbledore.
Whew. Quite the bit of foreshadowing there! I mean, you can't say something like that, and then not have it happen somewhere in the series. It's basic storycraft, Chekhov's Gun material.
But as far as I can recall, he never actually did trust Hagrid with his life. When Dumbledore's life was at serious risk in "Half-Blood Prince", he put his trust in
not Hagrid. And I can't recall another time where Dumbledore's life could seriously be said to be in danger, apart from the duel with Voldemort (which didn't involve Hagrid in any way). Dumbledore was driven out of Hogwarts twice, but his life could hardly be said to be in danger either time. He did trust Hagrid with important tasks, but did he ever actually trust Hagrid with his life?