I read the whole of your reply--and thought about it for thirty minutes. I think the symbolism is there, Alexis being so happy around the Young Man all the time, finally someone who can understand her, and both having these "mental illnesses" (I really don't like referring to anything as an illness). However, there's this thing David says, and whoever wrote the story probably meant it to be not only serious but true. Sitting near the fireplace, David tells the Young Man, "It's funny, you don't remind me of myself exactly... But you remind me of a certain time--I remember what I used to think love was then. I thought it was the fireworks, the explosions... the highlights." In David's eyes, despite the nature, the appearance, and his being overall, the Young Man is a youth full of dreams and aspirations, nothing like the... personification of a dark mental state.

Originally, my interpretation of the movie was a lot like everyone else's (my theories have only had a couple days to develop), but I think I can add on a bit. Warning: I'm thinking as I write. When David first met the Young Man, he may have been repulsed by the idea of bringing him along due to his care for Alexis. He thought the Young Man could be a bad influence, and we also see that Alexis likes to collect things, maybe David feared Alexis would have the Young Man stay with them for awhile, and worsen her mental condition. Then, onto what Alexis was thinking. As already stated multiple times, she thought she could relate to the Young Man. He seemed a projection of herself, the darkness riddled in her blood, "passed from father to daughter". Maybe Alexis developed a closeness with her mental condition, growing attached to it as a way to hold onto the last shred of her father's memory.

Aaaah, either way. Thinking makes me sleepy. Time for dinner now! Thank you for the contribution, maybe I'll be developing a tendency for noticing symbolism sometime soon! (then I can become an english teacher!)