TL; DR
Para mim, a única razão é que Sam amava Frodo. Ele sabia que tomar a coisa enquanto Frodo vivia seria a maior tortura para Frodo. Ele percebe que não pode tornar Frodo parte do anel sem causar um grande dano psicológico / físico a ele. No final, em vez de carregar o anel, ele carrega Frodo de costas para as portas do Monte Doom.
Versão longa
Sam pega o anel quando pensa que Frodo está morto
Isso significa que ele não é avesso a cumprir a missão por conta própria.
Quando Frodo foi envenenado por Shelob e Sam apesar de ter falecido, Sam ficou impressionado com a dor. Ele não podia deixar Frodo "morto", mas também entendia que tinha que continuar. Sua luta é descrita em Livro-IV (Capítulo-10) (ênfase minha),
'What shall I do, what shall I do?' he said. 'Did I come all this way
with him for nothing?' And then he remembered his own voice speaking
words that at the time he did not understand himself, at the beginning
of their journey: I have something to do before the end. I must see it
through, sir, if you understand.
'But what can I do? Not leave Mr. Frodo dead, unburied on the top of
the mountains, and go home? Or go on? Go on?' he repeated, and for a
moment doubt and fear shook him. 'Go on? Is that what I've got to do?
And leave him?'
'If I'm to go on,' he said, 'then I must take your sword, by your
leave, Mr. Frodo, but I'll put this one to lie by you, as it lay by
the old king in the barrow; and you've got your beautiful mithril coat
from old Mr. Bilbo. And your star-glass, Mr. Frodo, you did lend it to
me and I'll need it, for I'll be always in the dark now. It's too good
for me, and the Lady gave it to you, but maybe she'd understand. Do
you understand, Mr. Frodo? I've got to go on.'
'What? Me, alone, go to the Crack of Doom and all?' He quailed still,
but the resolve grew. 'What? Me take the Ring from him? The Council
gave it to him.'
But the answer came at once: 'And the Council gave him companions, so
that the errand should not fail. And you are the last of all the
Company. The errand must not fail.''Let me see now: if we're found
here, or Mr. Frodo's found, and that Thing's on him, well, the Enemy
will get it. And that's the end of all of us, of Lorien, and
Rivendell, and the Shire and all. And there is no time to lose, or
it'll be the end anyway. The war's begun, and more than likely things
are all going the Enemy's way already. No chance to go back with It
and get advice or permission. No, it's sit here till they come and
kill me over master's body, and gets It: or take It and go.' He drew a
deep breath. 'Then take It, it is!'
Então, Sam relutantemente assumiu esse fardo. Depois disso ele também teve que colocá-lo para escapar dos orcs.
Sam não é facilmente corrompido pela atração do anel
Quando ele percebeu que Frodo não estava morto (tendo ouvido o líder dos orcs falar sobre isso), ele estava além de si mesmo com raiva e toda a sua resolução anterior foi esquecida. Em Livro Seis (Capítulo-1) vemos que Sam só quer resgatar seu mestre e não é tentado por pensamentos de ser o mestre do anel.
He no longer had any doubt about his duty: he must rescue his master
or perish in the attempt.
As Sam stood there, even though the Ring was not on him but hanging by
its chain about his neck, he felt himself enlarged, as if he were
robed in a huge distorted shadow of himself, a vast and ominous threat
halted upon the walls of Mordor. He felt that he had from now on only
two choices: to forbear the Ring, though it would torment him; or to
claim it, and challenge the Power that sat in its dark hold beyond the
valley of shadows. Already the Ring tempted him, gnawing at his will
and reason. Wild fantasies arose in his mind; and he saw Samwise the
Strong, Hero of the Age, striding with a flaming sword across the
darkened land, and armies flocking to his call as he marched to the
overthrow of Barad-dur. And then all the clouds rolled away, and the
white sun shone, and at his command the vale of Gorgoroth became a
garden of flowers and trees and brought forth fruit. He had only to
put on the Ring and claim it for his own, and all this could be.
In that hour of trial it was the love of his master that helped most
to hold him firm; but also deep down in him lived still unconquered
his plain hobbit-sense: he knew in the core of his heart that he was
not large enough to bear such a burden, even if such visions were not
a mere cheat to betray him. The one small garden of a free gardener
was all his need and due, not a garden swollen to a realm; his own
hands to use, not the hands of others to command.
Seu coração não está corrompido pelo anel (novamente, o amor por Frodo desempenha seu papel).
Sam vê que o anel de controle tem mais de Frodo
Quando ele finalmente resgatou Frodo e deu o anel de volta para ele, ele ficou surpreso com o anel de puxar que tinha começado a exercer em Frodo.
'All right, Mr. Frodo,' said Sam, rather startled. 'Here it is!'
Slowly he drew the Ring out and passed the chain over his head. 'But
you're in the land of Mordor now, sir; and when you get out, you'll
see the Fiery Mountain and all. You'll find the Ring very dangerous
now, and very hard to bear. If it's too hard a job, I could share it
with you, maybe?'
'No, no!' cried Frodo, snatching the Ring and chain from Sam's hands.
'No you won't, you thief!' He panted, staring at Sam with eyes wide
with fear and enmity. Then suddenly, clasping the Ring in one clenched
fist, he stood aghast. A mist seemed to clear from his eyes, and he
passed a hand over his aching brow. The hideous vision had seemed so
real to him, half bemused as he was still with wound and fear. Sam had
changed before his very eyes into an orc again, leering and pawing at
his treasure, a foul little creature with greedy eyes and slobbering
mouth. But now the vision had passed. There was Sam kneeling before
him, his face wrung with pain, as if he had been stabbed in the heart;
tears welled from his eyes.
'O Sam!' cried Frodo. 'What have I said? What have I done? Forgive me!
After all you have done. It is the horrible power of the Ring. I wish
it had never, never, been found. But don't mind me, Sam. I must carry
the burden to the end. It can't be altered. You can't come between me
and this doom.'
O próprio Frodo explicou a ele que era seu destino destruir o anel e eu acho que Sam também viu que ele não poderia "pegar" o anel sem ferir Frodo (ele também não se sentia inclinado a fazê-lo)
A cena que você referenciou acontece em Book-Six (Capítulo-3)
'I can't manage it, Sam,' he said. 'It is such a weight to carry, such
a weight.'
Sam knew before he spoke, that it was vain, and that such words might
do more harm than good, but in his pity he could not keep silent.
'Then let me carry it a bit for you, Master,' he said. 'You know I
would, and gladly, as long as I have any strength.'
A wild light came into Frodo's eyes. 'Stand away! Don't touch me!' he
cried. 'It is mine, I say. Be off!' His hand strayed to his
sword-hilt. But then quickly his voice changed. 'No, no, Sam,' he said
sadly. 'But you must understand. It is my burden, and no one else can
bear it. It is too late now, Sam dear. You can't help me in that way
again. I am almost in its power now. I could not give it up, and if
you tried to take it I should go mad.'
Então, o próprio Frodo responde a sua pergunta. Ele finalmente sucumbiu ao poder do anel e Sam não pode tirá-lo dele pela força sem causar sérios danos a Frodo. Sabemos que Sam ama muito Frodo (e também é sábio demais) para tentar algo tão tolo. Assim, quando chega a hora, ele toma sua decisão
'Come, Mr. Frodo!' he cried. 'I can't carry it for you, but I can
carry you and it as well. So up you get! Come on, Mr. Frodo dear! Sam
will give you a ride. Just tell him where to go, and he'll go.'
