“Sam was cocksure, and deep down a little conceited; but his conceit had been transformed by his devotion to Frodo. He did not think of himself as heroic or even brave, or in any way admirable—except in his service and loyalty to his master. That had an ingredient (probably inevitable) of pride and possessiveness: it is difficult to exclude it from the devotion of those who perform such service. In any case it prevented him from fully understanding the master that he loved, and from following him in his gradual education to the nobility of service to the unlovable and of perception of damaged good in the corrupt. He plainly did not fully understand Frodo’s motives or his distress in the incident of the Forbidden Pool. If he had understood better what was going on between Frodo and Gollum, things might have turned out differently in the end. For me perhaps the most tragic moment in the Tale comes in II 323 ff. when Sam fails to note the complete change in Gollum’s tone and aspect. ‘Nothing, nothing,’ said Gollum softly. ‘Nice master!’ His repentance is blighted and all Frodo’s pity is (in a sense) wasted. Shelob’s lair becomes inevitable.
This is due of course to the ‘logic of the story’. Sam could hardly have acted differently. (He did reach the point of pity at last but for the good of Gollum too late.) If he had, what could then have happened? The course of the entry into Mordor and the struggle to reach Mount Doom would have been different, and so would the ending. The interest would have shifted to Gollum, I think, and the battle that would have gone on between his repentance and his new love on one side and the Ring. Though the love would have been strengthened daily it could not have wrested the mastery from the Ring. I think that in some queer twisted and pitiable way Gollum would have tried (not maybe with conscious design) to satisfy both. Certainly at some point not long before the end he would have stolen the Ring or taken it by violence (as he does in the actual Tale). But ‘possession’ satisfied, I think he would then have sacrificed himself for Frodo’s sake and have voluntarily cast himself into the fiery abyss.
I think that an effect of his partial regeneration by love would have been a clearer vision when he claimed the Ring. He would have perceived the evil of Sauron, and suddenly realised that he could not use the Ring and had not the strength or stature to keep it in Sauron’s despite: the only way to keep it and hurt Sauron was to destroy it and himself together—and in a flash he may have seen that this would also be the greatest service to Frodo. Frodo in the tale actually takes the Ring and claims it, and certainly he too would have had a clear vision—but he was not given any time: he was immediately attacked by Gollum. … Frodo too would then probably, if not attacked, have had to take the same way: cast himself with the Ring into the abyss.”
—J.R.R. Tolkien, The Letters of J.R.R. Tolkien, ”246” (Part of his reply to a reader’s comments on Frodo’s failure to surrender the Ring in the Cracks of Doom)