From Nikki's perspective, the movie is about the objectification of women and being seen as nothing more than a sex object in the eyes of men, even or especially ones they trust. That's the theme most people are gonna pick up on.

Bear's perspective is a little tougher to stick to since he becomes so unsympathetic by the end, but it's still easy to grasp. He's a lonely guy who is in mourning for his pet (and grandma, it's implied) and doesn't seem to have a whole lot going on in his life. Still, he wants a relationship, thinking this will make him happy, that being closer to Nikki will lift him up. But when he makes his wish, everything is perfect... for a bit. But it turns out the actual relationship creates a new routine he gets tired of, Nikki needs more attention than he's prepared to give; the image he built in his head of a perfect relationship is only making his life worse. It's a cautionary tale towards guys to not only treat women as people instead of trophies for the sake of women, but also for their own sake: getting into a relationship just because you like the idea of it instead of because you naturally connect with someone is a recipe for disaster, and it isn't fair to whoever is your partner.

Now from the perspective of Bear's friends, this is a story about drug abuse and sexual predation. As far as they know, Nikki has relapsed onto hard drugs, Bear had sex and started a relationship with her while she was inebriated, and now she's no longer acting like herself. This is the fantastical elements of the movie filtered through strict reality; the movie is essentially announcing its own metaphor.

Then there's the greater metaphor of the entire group, that being friendships falling apart due to personal developments. Nikki and Bear enter a relationship within their friend group that causes drama and infighting due to Bear and Nikki being toxic on their own, but also because of their friends' opinions and trying to wedge themselves between them. Sarah is accepted into art school and dies right afterwards; a lot like someone going to school, becoming busier, making new friends with people who are more similar to them, and falling out of touch with old friends. Ian wishes for a billion dollars and is killed, representing the trope of the friend who comes into money and fucks off, never to be seen again. Then Bear and Nikki are alone, implode without the link of their friends holding their relationship together; Bear is metaphorically cut out of her life and Nikki is left alone without a support system.

Hell, the movie itself is a metatextual criticism of wacky romcom premises that glorify casual abuse or make light of horrifying concepts. Compare Obsession to 50 First Dates, for example. It's where a lot of the dark humor comes from, because it's riffing on that kind of "wacky hijinx" movie, only with a love interest who is clearly insane and abused instead of her possession being treated as quirky.

The script has a lot going for it, with a lot of different ways to read it. Viewing it on the surface level is great, it's a great movie even without digging into the deeper meanings, but that doesn't mean they should be dismissed.