Over the years, Star Trek has introduced a plethora of amazing technology and a fantastic array of different characters. Due to each one of the main characters being distinctly different, with different personalities and desires, there are often times when they clash. When done right, the result is TV gold.
Onscreen drama between chapters fuels a fair number of plot lines in various Star Trek shows. There's the dichotomy between the Marquis rebels and the Federation in Voyager, and the Cardassians' conflict with the Bajorans in Deep Space 9. However, sometimes tensions appear on a more personal scale. One of the most consistent, but often overlooked, rivalries from the entire franchise however has to be that between B’elanna Torres and Seven of Nine in Voyager. But why exactly did they fight so often?
The answer, like many questions about the Star Trek franchise, is two-fold. One explanation exists in-universe; the other, behind the scenes, reflective of the real world outside the fiction created by the show. In this case, the out-of-universe explanation has a lot to do with the unwritten rules of 90s TV shows. More often than not, writers and producers would force the two conventionally attractive female characters to hate one another.
There is a lot more to these characters than just being eye candy, of course. Torres is a powerful, smart engineer constantly battling between her overly emotional but powerful Klingon side. Meanwhile, Seven weaves a tale all about what it means to be human and the rediscovery of her own humanity. However, it's still unfortunately fair to say that a large part of their intended purpose on the show was sex appeal. The skin-tight ‘catsuit’ worn by Seven and the awkwardly long sonic shower scene with Torres are prime examples to this point. As such, according to the writing tropes of the time, it was inevitable that they would clash.
The less sexist reason however, which acts as an in-universe explanation, is bound to their very different personalities and the historical context of the show. Voyager is set just a few years after Wolf 359, a disastrous defeat of the Federation from the at the hands of the Borg. This was a devastating attack, and many lives were long all to just one powerful Borg cube. With this comes a lot of trauma and generational fear. Citizens of the Federation, such as Torres, have an overwhelming anti-borg sentiment. They are still fearful of Borg and the threat they pose. For Torres, a character who has trouble hiding or controlling her emotions, her dislike of the ex-borg Seven comes out more loudly than that of her peers, even those who may share the same fears and unintended hate towards Seven.
Their personalities are very different, but oddly similar at the same. Neither of them are particularly good socializers. Torres constantly carries a chip on her shoulder, and has one of the shortest fuses in the franchise. She also has an inferiority complex that comes out fairly often. Seven, meanwhile, is dealing with a lot of trauma herself, with her separation from the collective taking its toll on her state of mind. She also doesn’t quite know how to act like a ‘normal’ human, her mannerisms remaining distinctly Borg. Every personal interaction is an accumulation of leant behaviors and doing what she has been told is the right thing to do, and she gets it wrong all the time. This makes her incredibly arrogant and triggering for almost all the crew, and she accidentally insults people constantly. So when paired with Torres and her temper, it is a recipe for disaster.
One other possibility comes from a place of starship logistics, and would affect any engineer. When Seven came aboard, she inadvertently created a large amount of work for Torres, as she kept introducing more and more Borg technology for the ship and the crew to use. It is Torres' responsibility to make sure all this works perfectly in conjunction with the Starfleet components, throwing her in the deep end a lot of the time. Torres was also responsible for clearing up the mess that the still-Borg Seven of Nine and her fellow drones caused when they partially assimilated the crew and ship. It was a huge clean-up that Torres had to deal with, personally blaming Seven even though it was not technically her fault.
Voyager follows a lot of common TV tropes, and for a lot of fans this results in the show falling flat in comparison to the others in the franchise. While the dysfunction between Seven and Torres is a prime example of an outdated trope, there is more to it than that. There are plenty of in-universe explanations for their dislike of one another. Either way, the most important takeaway is that if the two are in a room together, make sure to stay well out of the way.