I also think the movie does an inadequate job of explaining just who Boromir is and why he and Aragorn have a built-in rivalry. He is in practical terms the heir to the throne of Gondor. If Aragorn returns, he will displace Boromir's father and his entire family from their position of power.
If you know this, Boromir's dying declaration of fealty to Aragorn packs a lot more punch.