There are several reasons why we could call Sam the main character in LotR, but the primary reason is found in his relationship with the reader. Let me explain. Nearly all the characters in The Lord of the Rings have either had interactions with different peoples/races/creatures or at least know a fair amount of the world at large. They are knowledgeable about the world they live in. The primary exception to this rule are Hobbits. Even so, Bilbo, Frodo, and arguably Merry cannot be truly labeled 'innocent'; Bilbo for obvious reasons, Frodo because he was raised by his uncle, was friends with Gandalf, and possibly met elves before starting his quest, and Merry because he knew enough of the Old Forest to attempt a short-cut. Sam on the other hand is entirely oblivious to the wide world of Middle-earth. He grew up hearing and dreaming about Old Bilbo's stories but elves and dragons were always a distant reality, a kind of fairy-tale. The first-time reader of The Lord of the Rings is in the exact same position. Most people have dreamt of dragons and mythical creatures but they've never stepped in a world quite like Middle-earth. As the story progresses the reader and Sam are exposed to the same events, sights, and characters for the first time. In other words, Sam is the most identifiable character.
Likewise, because Sam is one of the most oblivious people at the start he learns and experiences the most. At the beginning he's a simple, "genuine" hobbit who's never gone farther than the "Woody End". By the time he comes home he has more stories to tell than Old Bilbo. He's a veteran who has travelled thousands of miles, narrowly escaped numerous pursuants, watched wars, walked into Mordor, and seen the Ring of Power destroyed. You could say many of those things of other characters, but none of them change as dramatically of Sam. Frodo for one does all these things and more, admittedly. But his character, outlook on the world, and his knowledge of Middle-earth doesn't change as much as Sam's. Before leaving home Frodo knew of lands they were journeying to, he was more fully aware of the dangers entailed by their quest, he knew more of the creatures and races they would meet along the way, and he knew that he was making a great sacrifice. For Sam every new valley was a step from home. His character went the furthest.
Third, The Lord of the Rings ends with Sam. Read again the closing paragraphs from The Return of the King:
Then Frodo... went aboard; and the sails were drawn up... And the ship went out into the High Sea and passed on into the West... the grey rain-curtain turned all the silver glass and was rolled back, and he beheld white shore and beyond them a far green country under a swift sunrise.If Frodo was intended to be the main character this would be a perfect ending, but Tolkien doesn't finish there, he goes on:
But to Sam the evening deepened to darkness as he stood at the Haven; and as he looked at the grey sea he saw only a shadow on the waters that was soon lost in the West. There he stood far into the night, hearing only the sigh and murmur of the waves on the shores of Middle-earth, and the sound of them sank deep into his heart....
At last they [Sam, Merry, & Pippin] rode over the downs and took the East Road, and then Merry and Pippin rode on to Buckland; and already they were singing again as they went. But Sam turned to Bywater, and so came back up the Hill, as day was ending once more. And he went on, and there was yellow light, and fire within; and the evening meal was ready, and he was expected. And Rosie drew him in, and set him in his chair, and put little Elanor upon his lap.
He drew a deep breath. 'Well, I'm back,' he said.
The Return of the King, Book VI, Chapter 9, "The Grey Havens"With those words Professor Tolkien ends his 1,000+ page masterpiece, with Sam returning home. It's an important point: this tale ends (and really continues) with Sam Gamgee. Indeed, Tolkien originally wanted to elaborate more on Sam and his daughter Elanor. (Letters No. 173) Why? To make the point even clearer, that Sam is the main character of The Lord of the Rings.
Who do you think the main character in The Lord of the Rings is? Is there a different answer for the films? Please feel free to discuss below!