Friends that I have talked to who are not readers of the book don't "get" why Frodo had to leave the Fellowship at the end. They don't realize, without the book, that Frodo feared that the ring would ruin all of them eventually, that Boromir was just the first to fall.I thought Frodo covered that when after fleeing Boromir, then takes off the ring, he falls off the seat and Aragorn comes up behind him. Frodo says "It has taken Boromir" and Aragorn asks "Where is the Ring", and moves to help Frodo. Frodo backs away and asks "But can you save me from yourself" Then Aragorn folds the ring into Frodo's hand, telling him he would have gone with him to the end. Aragorn understands Frodo's concerns and is giving Frodo permission to go on his own. That's why Frodo tells Aragorn to take care of Sam because he won't understand.
I guess I understand more because of the books, but I only read them once each before the movie. I haven't read them as much as some of y'all have.
Now that particular scene wasn't in the books, and perhaps makes it seem even more confusing. Aragorn passed the test, and said he would have gone to the end. They couldn't figure out why Frodo would turn that down...
To me it was covered too... but I had to do a lot of explaining to my friends. A lot is said with an expression to me, that went right over their heads. Just a little thing... I am happy to hear about these cool new scenes.