Now that I have almost finished reading The Lord of the Rings, one of the differences between the books and movies puzzles me more than any other: In the books, Aragorn always intended, and expected, to claim the throne of Gondor - he even carries the Shards of Narsil around with him. He probably wants to become King for other reasons as well, but he also has to become king before Elrond will allow him to marry Arwen. No one who knows him ever doubts that he will be king, and Narsil is remade before he even leaves Rivendell.
In the movies, he is reluctant to claim the throne, to put it mildly. He is more committed to his role as a Ranger. He barely even touches Narsil before it is reforged. He avoids giving the impression that he is interested in the throne. No one who knows him, except Arwen and perhaps Gandalf, thinks he will ever be more than a Ranger. Elrond only reforges Narsil after Arwen forces him to choose between fixing the sword or watching his own daughter die. Aragorn only seems to come around to the idea of taking the throne after the story, and the war, are almost over. Arwen's emotional blackmail of her father was also emotional blackmail of Aragorn - if he hadn't accepted the sword, and by extension, the throne, he'd have a dead girlfriend on his conscience.
Overall, in my opinion, Jackson made Aragorn seem hesitant, indecisive, weak, and reliant on the influence of others to make decisions. Imagine being a citizen of Gondor and knowing that your king wasn't sure he wanted to be your leader. In the books, Aragorn has his moments of doubt, but always regarding how to go about the quest of the Fellowship, never about whether or not he would claim the throne one day.
I assume Jackson was thinking about a character arc, character development, leading the audience along the path of Aragorn's rise to greatness, etc. But has he ever commented on this subject explicitly?