Samwise Gamgee likes living the simple life. He loves drinking beer with his buddies. He likes daydreaming about his sweetie-pie Rose. He likes his neighborhood, and probably aspires to sit on some Shire city council when he's old and gray. He has no real interest in what lies beyond his hometown.
But, unfortunately for Sam, this makes him exactly the kind of person that filmmakers like to force into adventures.
And, to be fair, it's Sam's fault that he's picked to go on the journey to Rivendell: he's an epic snoop. On that fateful night as Gandalf council's Frodo about the Ring, Sam just happened to listen in outside Frodo's window. When Gandalf picks him out of the bushes, Sam is terrified that Gandalf might turn him into something "unnatural."
We know Gandalf's a capital-G Good Guy, so he doesn't turn him into something unnatural. But he does make him do something unnatural… especially for the home-loving hobbits. Gandalf makes Sam Frodo's traveling companion.
Before they've hardly started their journey, Sam is already confronted with the newness and scariness of the world outside the Shire.
SAM: This is it. If I take one more step, it'll be the farthest away from home I've ever been.
Sure, reaching the outskirts of the Shire seems like a big deal at the time, but Sam might find this statement laughable if he were to remember it while on the slopes of Mount Doom.
Sam gets psyched about seeing fulfilling his lifelong dream of seeing elves, but even once he gets to Rivendell he's already thinking about heading back:
SAM: We got the Ring this far to Rivendell and I thought... seeing as how you're on the mend, we'd be off soon… off home.
And don't expect this homesickness to wane during the Lord of the Rings trilogy. While Sam certainly changes during his journey with the fellowship, there are parts of him (the stereotypical hobbit-ty, Shire-loving parts) that will stay the same.
But Sam ain't just a reluctant traveler and homesick hobbit-hole dweller. In fact, those are secondary aspects of his character. The #1 most important character-defining trait of Sam is his loyalty. Dogs might be a man's best friend, but Sam is definitely the best friend a hobbit could hope for.
He's always looking out for Frodo. Even at the very beginning of the film, he stops Frodo from putting on the Ring as the Nazgûl closes in to their hiding spot. And even before they're fully aware of the danger of the Nazgûl, Sam is already worried about losing Frodo in a cornfield. Like he explains to Frodo,
SAM: It's just something Gandalf said: don't you lose him, Samwise Gamgee. And I don't mean to.
Sam sticks with this promise until the very end (although really we have a feeling it's got more to do with his loyalty to Frodo than a command from Gandalf). By the end of the film, Sam forces Frodo to take his company to Mordor by attempting to swim to Frodo's boat… even though it appears that Sam missed out on swimming lessons at the Shire Community Swimming Pool.
Sam will do anything to stay with and protect his bud. And even as Frodo's depression and isolation deepens in The Two Towers and The Return of the King, Sam's loyalty stays steadfast.