I have a question... 
Seeing as how I've watched the ROTK:EE about six times so far, something still bothers me, and I was talking about it last night with my friends while we watched it...
Why did Frodo have to leave to the Undying Lands? One friend said "Well he's seen to much...", which to me isn't really an explaination. I mean Sam has been to the same places that he has and Merry and Pippin undoubtably saw more terrible things in the middle of a war...
Another friend suggested that since he had been touched by Sauron, he had some of his evil inside of him, and it could come back... But I feel that isn't explicitly said, so that's not a complete explaination to me.
Yet another person there said something to the effect "Well when he got stabbed he got a little bit of evil in him, and if he doesn't leave with the elves, the magic they used to keep him alive will go away and he'll die." Well to me isn't adequately explained either and doesn't stand up as an explaination. I mean if that were true then wouldn't that mean anyone who was wounded by a Morgul weapon, like that wielded by the Witch King, (i.e. Eowyn) would die as soon as the person who healed it died?
It was a combination of things, IMO. Perhaps the best way to explain it is that Frodo was so wounded in body and spirit by his long struggle with the Ring, and the other wounds he'd taken--especially being stabbed by the Morgul blade--that neither time nor mortal magic could heal him. He wouldn't have dropped dead if he had stayed in the Shire, but neither would he have been healed and able to live a normal, happy life. Personally, I think the most important wound was to his spirit, though the book also says explicitly that the pain from the Morgul knife wound never really leave him.
Eowyn's arm was broken by the Witch King's mace, which is quite different, I think. She was never in peril of being turned into a wraith like Frodo was.
One thing I was thinking about was that it was some sort of 'reward'... Like since he carried he ring in order to destroy it, then he was to be rewarded with everlasting life (kind of like Bilbo). But maybe it's because I never quite understood the Undying Lands, other than the literal connotation of the name, that I'm having this problem...
Good call. It definitely
was a reward for what he endured to save ME. Part of that reward was the chance that the magic of the immortal Valar could heal the wounds that could not be healed if he stayed in ME.
I won't go into too much detail about the Undying Lands, 'cause there's a lot you could say, but the basics are this. "The Undying Lands" are the continent of Aman which lies across the ocean to the west of Middle Earth. The immortal Valar, their servants the Maiar (of which Gandalf is one) and many elves live there. For the elves, who are also immortal, Aman is considered to be thier 'true home' in a sense, and all elves are welcome to travel there. It is not accessible by normal means, but elves can sail "the straight path" and find their way there.
In general, mortals are not allowed in Aman. (Which is the core of the story of the Downfall of Numenor, but that's another tale.) However, Frodo and Bilbo--and some others later, it is told--were given special dispensation to go there as a reward for their service. Though the name "Undying Lands" kind of implies it, Tolkien was clear that mortals who travel there do
not become immortal themselves. Frodo will live out the rest of his life there, hopefully healed and happy.
So I was thinking, since some folks here obviously have knowledge of the books and the movies which surpasses my own, that you could give me a little insight into the reasoning behind Frodo leaving. Maybe it's something that is explained better in the books? Maybe it's something that I missed in the movies?
Yes, it is explained somewhat better in the books, but some of it you still have to read between the lines. I'll give PJ credit, he tried to put some dialog in there that would help explain it, but it's kind of a subtle concept so I'm not surprised that it wasn't terribly clear to the non-book readers in the audience.
Hope I helped some.