" And every evening, when Frith has done his day's work and lies calm and easy in the red sky, El-ahrairah and his children and his children's children come out of their holes and feed and play in his sight, for they are his friends and he has promised them that they can never be destroyed."
Towards the end of Watership Down, months after their adventures, Hazel happens upon a mother rabbit telling her kits a story ostensibly starring the rabbit's legendary hero El-ahrairah. It gradually dawns on him that the tale is really his story: the tale of the escape of his True Companions from Sandleford and journey to Watership Down. Many tears of joy ensue.
Also the bit where Bigwig is fighting Woundwort and tells him that he will defend that run until he's dead. And this from the rabbit who, in the beginning, was objecting to Hazel's leadership.
Bigwig: My Chief Rabbit has told me to defend this run and until he says otherwise I shall stay here.
"Oh fly away, great bird so white...you know, he made me feel I could fly, too." And so Kehaar leaves, if only for now.
At the end, when an old and weary Hazel suddenly starts feeling better than he ever had in his whole life... then turns and sees his own dead body laying in the grass. He feels just a fleeting moment of regret, then happily bounds off to join all those who've gone before.
When Strawberry joins the band of travelers. Silver sneers at him that they don't like rabbits who betray them, and he'd "Better go back to Nildro-Hain", his mate. Strawberry gives a cry of pain, and finally manages to whimper two words: "The wires." Having seen firsthand how much Nildro-Hain meant to Strawberry, the news that she has died—so suddenly and unnecessarily—is like a knife in the heart of all readers. The fact that his mate's death hurt him so badly that even someone like Strawberry, so jaded to constant death that he completely shrugged off the loss of his close friend Kingcup, couldn't bear to live without her made this troper tear up. Badly. The heartwarming part is Hazel's immediate response.
"Don't say any more. You can come with us. Poor fellow."
"I'll come with you, Hazel-rah."
Anything involving Pipkin. Specifically, the above quote and the fact that he's willing—actually, he begs to be allowed—to risk his own life remaining in the Honeycomb to try to wake Fiver, rather than burying himself in a burrow with the others.
Bluebell caring for his captain, and telling the frightened does a story during the fight, to keep them from panicking.
"Oh, Frith on the hills! He must have made it just for us!"
"Frith may have made it... but Fiver found it."
Bigwig and Hyzenthlay's interactions in Efrafa are ridiculously cute. Just the way they reassure each other and keep each other going...
"I would rather be here, now, as we are, than to never have left Efrafa."
Hazel's mouse, returning to warn him about the Efrafans. Even though he didn't realize what was going on, he saved their lives. After Bigwig shrugging off the idea of rescuing the mouse in the first place, it's great to see how useful their little friend really was.
Fiver taking care of Hazel while the latter was recovering from being shot in the leg.
In the film, Hazel's prayer to Frith: "Lord Frith, I know you've looked after us well, and it's wrong to ask even more of you. But my people are in terrible danger, and so I would like to make a bargain with you. My life in return for theirs."
Adding to that, Frith's reply, highlighting the fact that these rabbits are not the only ones who can be heroes, and that people like them live and die every day.
"Not a day goes by but a doe offers her life for her kittens, or some honest captain of Owsla his life for his Chief Rabbit. Sometimes the bargain is accepted; sometimes it is not. But there is no bargain here—for what is, is what must be."
That bears thinking about as well, and crosses over with a CMOA. Frith, the all-powerful god, is essentially saying that in this, Hazel creates his own fate. Neither Frith nor the Black Rabbit interferes. Which means that the miracle they pull off? That was them. No god made that happen. It was their strength, their courage, and their spirits and ingenuity alone that saved Watership. Hazel-rah, indeed.
Is it a mystery why he was asked to join Frith's Owsla?
Not Frith's Owsla - El-ahrairah's. Which is arguably even better. Also, that line about bargains? In the book, the Black Rabbit says that to El-ahrairah. Hazel is in exceptional company there.