The Farmer flashing back to his happy childhood and sparing a tree he was about to cut down.
In the episode 'Two's Company', realizing that Shaun and Lola have fallen in love in each another as Lola was about to be taken home, Bitzer switches Lola in the truck with a decoy at the very last minute, so Lola gets to stay.
When the invitations are ruined before they can reach any guests, the disguised flock gives the farmer the birthday party of his dreams.
In the Christmas Episode, it's shown that the Farmer would be celebrating alone, so the flock and Bitzer manage to completely decorate his living room and knit him a sweater out of their own wool for a Christmas present.
And prior to that, the Farmer dresses as Father Christmas and distributes presents to all the animals on the farm.
The farmer saving Shaun from plunging into the ravine.
Shaun and the others singing a sheep version of "Feels like Summer" to Timmy.
Slip the stray finally finding someone to take her home, the bus driver with the same snaggletooth smile as her.
When the farmer doesn't return after his caravan rolls into the city, the other sheep start missing him, resulting in Shaun going on his own to find him.
When it seems that Trumper is about to throw them all into a quarry, Shaun, Bitzer and the rest of the sheep stop trying to escape and start hugging the Farmer despite knowing he doesn't remember them. It's very reminiscent of frightened children hugging their father, hoping that he'll protect them.
How the farmer finally regains his memory? He sees a reflection of himself in the shed window, with Bitzer and the sheep hugging him, and flashes back to when he was younger and they were all infants. Upon remembering he doesn't even register the terrible danger they're all in, just that his animals are frightened, so he immediately starts comforting them.
The Farmer seems just about ready to attack Trumper for trying to harm his animals once everyone is safe.