[Iris Alantiel]

I think the ultimate fate of all four hobbits goes to show exactly how much the quest has altered each one of them, and in Sam's case, I think the biggest indicator of how he has change is at the very end of the story, when he has to go on with his life without Frodo. Through the entire story we hardly ever see the two of them apart; Sam is pretty well always at Frodo's side. The relative calm with which Sam begins his life without Frodo indicates some of the change that has taken place in Sam.

I'm not sure if this was in Tolkien's mind at all when he wrote the books, but I think Frodo's departure into the West highlights how much Sam really has grown as a character. In the very beginning, Sam is there out of blind loyalty; his key characteristic is his loyalty to Frodo. That characteristic remains intact throughout the story, but as time progresses, he becomes less like a blind follower in Frodo's life. The fact that Frodo and Sam differ over Gollum - Frodo pities him, while Sam distrusts him - demonstrates the fact that Sam, though still loyal to Frodo, has grown enough to express disagreement with him. After the incident with Shelob, when Frodo is dead, Sam attempts to separate himself from Frodo to destroy the ring; he is unable to do so, but he gets points for trying. Sam's personal growth culminates at the end of RotK, when he parts from his beloved Frodo forever, very sadly, but with a fair amount of grace and strength. He transforms from being Frodo's faithful follower in the very beginning to an independent hobbit with a life of his own and great emotional strength and character.

I realize that I kind of got off topic a little bit from the original question of why Frodo felt he had to leave ME, but I hope maybe this could point to a reason why Tolkien might have written it that way.