They tell him that house-elves are useless and worthless unless they serve their Master. They tell him that self-respect is not something house-elves are allowed to have. They tell him that the magic surging in his veins are not for his own personal use, but as a tool to serve great and powerful wizards.
They told him all wizards are the same, they are all good and meant to be served.
And he decides that they're wrong and he breaks free.
His name was not 'elf' or 'stupid' or whatever those Malfoys thought of– his name was Dobby.
His masters are not good, they are bad and should not be served, but alas; he is bound to them by magic.
Dobby has seen generations of elves get mercilessly sacrificed for the Dark Lord, because they do not think for themselves. Dobby knows this and he constantly thinks- something that he does not, should not do for he was just a house elf.
He has read all about Harry Potter and decides that Harry Potter was a good wizard.
And Harry Potter was.
Harry Potter is now his best friend, but to Harry, Dobby was merely a house elf. But Dobby was fine with that, because Harry Potter cared about him and he was worth dying for.
So when the bad witch throws the sharp dagger in fury towards Harry, Dobby decides for the last time to change history. He has seen the knife flying towards Harry and does not think as he jumps in front of him.
It goes through his mortal body and stops at his spine with a dull thud.
The wound hurts, it hurts more than anything he's experienced before. Even when he was with the bad wizards, the beatings were not as painful as this knife in his chest.
Dobby is calm as blood pours out of the wound which was very unlike him. Usually, Dobby was incredibly squeamish around his own blood, sometimes sobbing in fear when he saw it well out of his whipping wounds or his burns. It feels... exhausting.
The house elf struggles to stay awake, and notices that the sky is above him now. All he wants to do now is sleep, with the sky above him, Harry Potter's arms carrying him, and having this peaceful feeling for the first time in years.
He remembers when Harry had given him an old pair of yellow mustard socks. Dobby still wore them everyday, underneath his own knitted socks. He catches a glimpse of the snitch socks that he has once knitted on Harry Potter's feet and he smiles.
It hurts so much and all he wants to do is sleep. But losing Harry Potter and the thought of it hurts more than that. As the light left his eyes, he notices that Harry Potter's eyes are moist and there is a pained howl that seems far away.
There are tears and Dobby thinks for the first time that Harry Potter, the boy-who-lived, may actually mourn him, the useless House-Elf.
Dobby dies knowing that Harry Potter was worth dying for.