gif posted here on fanpop by katekicksass
So Rowling never really discussed why Bellatrix is so obsessed with Voldemort. And I constantly ponder upon how anyone could find that no-nosed snake-man-boy attractive in any way. With that said I wanted to post a few of my theories as well as some of the interesting ones I've seen across the net. Once again, as stated, these are just theories and speculations. No more, no less.
First and foremost I think it's a pretty safe bet to say that she loves him or at least started to love him based on the fact that they share similar ideologies. Both loved the idea of pure-blood supremacy. A lot of bonds form from sharing one common ground. But that begs the question; was it her hatred of muggles/muggle borns that had her attracted to Voldemort, or was it Voldemort who instilled these beliefs in her (rather, solidified them seeing as--based on family--she probably already held them).
One theory that I came across is that she was under the imperius curse and no one noticed. Like it made her so frantically devoted to Voldemort that it kind of just slid under the radar. As for the how's I've heard people say that since she died we'd never know; after the war a lot of the Death Eaters avoided punishment by saying that they were under the curse (if I remember right). However there are still holes in that Bellatrix went to Azkaban for 15 years the first time he died. And for that, I don't know if I agree with this one, but it's a neat theory.
Similarly I've seen a handful of people say that she was slipped a love potion. Again, I don't agree with this one but it's an interesting thought.
I personally am inclined to think that she only loved Tom Riddle and that she doesn't truly care for Voldemort though she wants to. So she kind of forces herself to love the man-snake Tom became, which is why she comes on so strong. I mean Tom was kind of good looking. Furthermore I like to wonder if perhaps she and Tom were actually lovers before he split his soul and completely lost the capability to love. On that line of thinking her trying to make herself love Voldemort would be even more interesting. And (now this is getting a bit far-fetched lol) perhaps it gets more complex in that she continued to love Voldemort in hopes that Tom would resurface--even though deep down she knew Tom was gone, the human and loving part of Voldemort was gone.
Another idea I thought up was that, perhaps he reminds her of someone else she loved & possibly lost. Like she had a childhood love or something of the sort who had departed in some way or another be it death or a simple break up. Bellatrix (though I love the woman deeply) seems a tad clingy in someways. She's definitely got an obsessive personality. I could see her seeking out someone similar to a first love and latching on to him, pretending that he is the same person as her old lover.
I've also come across one person offline who believed that Bellatrix loved him because he could offer her protection. This is another one I don't agree with as I feel Bella is way more than capable of taking care of herself.
On the other end of the spectrum, painting Bellatrix as the feisty and wild woman she is, maybe it's a rebellion thing. This one also reaches back into Voldy's days as Tom. What if her love for him comes from her reluctance to marry Rodolphus. Many fans (myself included) believe that her marriage to him was arranged and out of pureblood duty. So what if she latched onto Tom as a sort of middle finger to her parents (who I presume were behind the marriage). Bellatrix definitely seems like a woman who doesn't like being told what to do, so infidelity would be the way to go.
At one point I also thought about attachment issues & approval seeking. I've known many people who have had father issues or poor relationships with their parents. A good few of the sought out someone like their fathers in hopes of gaining fatherly approval vicariously through a lover. I have wondered if this was the case for Bellatrix. I mean her family line was pretty messed up and I would imagine (given the time) that her family was very uptight and hard to please. So perhaps Voldemort reminds her of her father and if she gets his love/approval she gets her father's love/approval.
I feel like this could work even without factoring a father in. Perhaps Bellatrix is just starving for approval in general and she's fixated on getting it from Voldemort.
As for attachment issues, if she was poorly attached to her parents it is more likely that her future relationships would have ill attachments as well. Similarly to the above, it is possible that an unstable attachment to her parents is responsible for her unhealthy attachment to Voldemort. Since she couldn't get it right with them she'd latch onto Voldemort instead.
That or she had just somehow come to learn damaging attachments without her parents or is obsessive by nature and just latches onto people she has eyes for.
One of my favorite theories though is probably one of the most out there of them. I've actually seen quite a few people speculate that Bellatrix was a horcrux. These people believe that she is like Harry; a horcrux Voldemort never intended to make. One that he wasn't aware of. And something she herself didn't know she was. What makes this more interesting is that she died before
Voldemort just as all the other horcruxes so it can't be disputed in that way. In fact people used her death to back up this theory; "Bellatrix's gloating smile froze, her eyes seemed to bulge: for the tiniest space of time she knew what had happened, and then she toppled, and the watching crowd roared, and Voldemort screamed."
Voldemort definitely isn't one to scream at the loss of alleys. While it could be a cry of rage because she was such a powerful alley, it clearly was because he lost someone he loved/cared about. Following the thought that he was unaware of Bellatrix being a horcrux perhaps he screamed because he knew something more than just Bella had been lost. Or it was a cry of pain similar to his feeling of weakness at the loss of his other hocruxes.
But let's say for a moment that he knew Bellatrix was a horcrux; he'd definitely be screaming in rage then! That would also make the fact that he left the Hufflepuff Cup in her care even more interesting.
Often times the people who hold this belief seem to use Harry as proof as well; Harry had a connection to Voldemort (for example his parseltongue and his scar hurting when Voldemort was around as well as the visions he got). People have stated that Bellatrix's connection came in the form of her frenzied loyalty and ability to do whatever he said without question. She loves Voldemort because she literally has part of him inside of her. She desired to be his soulmate because she literally was his soulmate.
The question is when would he have made her into one then? When thinking about this theory I like to think that she was one of his first; she was lingering around when he made his first attempt at creating one and because of his inexperience part of her was transferred into her instead of the desired object. When it didn't work he didn't think anything of it and simply tired again. Other people like to think that she became a horcrux later on, that she was a fluke just like Harry.
This is another one that seems very far-fetched to me but I love thinking about it and it makes for great AU fics.
gif found on Bellatrix's wikia page.