Camille was too needy!
She defined her life entirely through Sullivan, who himself was this sort of shiftless, immature pseudo-artiste; truly an archetypical "young love". When Sullivan goes to see her, they almost always end-up having sex. Camille, for Sullivan, is a kind of indulgence, not so much another person; for him, she's not a fully-realized human-being, but merely fun.
Sullivan is Camille's whole world. But at the same time, again typical of young love, he is basically a penis, which she enjoys. When he leaves her, really what's happening is that she no longer has his penis to play with, or his hair to stroke. You never see them take care of each other in a mature, anticipatory manner. Meaning Sullivan never cooks for her, he doesn't invite her over to his mother's house. When he leaves for South America it's more of a fait accompli for him, rather than something he's given serious consideration.
He gives no thought to discussing something that big with Camille. For him, it's basically a fact of life that she has to accept. If he truly loved her as a human being, and not merely for the sex, he would have at least taken her wishes into consideration. Yet, Camille's reaction to Sullivan's parting is tantamount to the grief a little girl feels when her puppy gets run-over. Sure it hurts, but it's not the end of the world.
I've personally seen this way too often. Some grungy, but handsome guy with no real goals in life, other than some ill-thought pursuit, like being in a band and such, decides to leave a girl for whatever reason. Then it's as if her whole world came crashing down, when really what happened is that she no longer has his penis to play with. Her whole life was defined, not just through her boyfriend, but with the need to take care of him and "fix" him. She believes if she loves him enough, he'll stick around. But again, with young love, or immature love like that, the lovers do not really consider the fully human implications the other person represents. So that for the boy, the girl is a play-thing; for the girl, the boy is her puppy.
With young love, there's no room for worldly considerations. To the lovers, it feels like some sort of transcendental life-altering experience, meanwhile they've never had to spend the night watching over the other in a hospital, or dealt with the death of the other's parents, etc. In other words, for the lovers everything's possible: it seems like they'd move the Earth and the heavens for their love, except when it truly matters--we'll be together forever, so long as you don't interfere with my video games, or hanging out with my friends, or indulging my petty fantasies.
But truly, though, the final scene was beautiful. Years later, I take it, when Camille is with the older man at the cabin and she goes for a swim; the hat Sullivan gave her is blown-away into the water, and down the river. There was so much in that final scene. The world is ephemeral--you can't hold on to the past no matter how hard you try. Moreover, you must not hold on to it, you have to move on.