but the others from the onset were addicts. They love each other, they love their mothers, but they are still addicted to drugs and I find it difficult to sympathize with them.
Yes, she did drugs, but goddammit, she was still a human and capable of making mistakes. I understood her problems, she wasn't just an addict "just 'cause" like Aronofsky paints Leto, Connelly, and Wayans.
I find that the choice to not show why they do drugs is intentional. He's not depicting them as addicts "just 'cause," but rather saying that it shouldn't matter why they're addicts. Once the addiction takes hold, it dominates their lives and the reasons for starting have little to do with the reasons for continuing. I agree it can be hard to sympathize with the characters for the first part of the film, since I, like you, have never had an interest in drugs, but to me that made it all the more impressive that I could feel for them once things really got bad and the mask of happiness was lifted. If he'd set them up as using drugs because of a bad family life or abuse, or job loss or whatever else it would feel even more manipulative than some people already accuse it of being.
You mention a few times that the message seems to be "if you're poor, you'll end up doing drugs. So... don't be poor?" but it's actually the converse. If you use drugs they'll take such a hold on your life that you'll end up poor no matter how much money you make. I don't remember anything in the film to indicate that they were particularly poor before their drug use, every hint of poverty is directly related to needing money for their fix. When they reach their high point in the film they've got money and yet that doesn't stop them from using drugs, so not being poor isn't the answer.
Well, the film is pretty shallow if the only instance I truly care for the characters is when bad things start happening to them because of their addiction. I don't think there's enough backstory for any of the characters, except Burstyn's, that give any real reason for the characters to be onscreen other than the fact that they are drug users and that their lives are defined by the drugs that they are doing. While that's true for most drug users, they usually had some sort of life beforehand. No, Requiem for a Dream shows us a addict from the start and him going from bad to worse and there's little reason to care for the younger characters in the film. I feel like this conversation may get us to a brick wall where none of us can really permeate. You either care for these characters or you don't. I find everyone aside from Burstyn to be rather unsympathetic. You say that poverty is not a reason for their drug addiction, but what else is, hm? And there are many examples within the film that the characters are poor - they don't have jobs, they don't live in a great part of town, he steals the tv in the first scene so he can buy some heroin - of course they're poor! But if there's no reason for why they started doing drugs, then we can wave our fingers at the characters and tell them that they brought it upon themselves. If that truly is the case, then, Aronofsky can only rely on the images of torture to make me ache for the characters. If all he's relying on is visuals, then all I care for is visuals, not the characters deep down.
BAH BAH BAH
The film's goal is to show the horrors of drug addiction. In that regard, it succeeds. But it's really just a piece of propaganda fiction, right? There's nothing beyond that, in my opinion.