As someone who studied Voyager from a writing aspect (and pitched to it six times), I found the relationship almost as abrupt as Chakotay/Seven.
I think it's easy, afterward, to pick out moments from previous seasons and believe them to be "the beginning", but I honestly don't believe they were intended as groundwork. We could just as easily find moments to set up almost any other pairing, had it happened.
The Tom/B'Elanna relationship (in terms of series character arcs) just really hit me as being from left field.
--Ted
To be totally honest, I never felt invested in Paris/Torres romance. But I guess that it began for Tom, in "Macrosom", when he flirted with B’Elanna and then they were sick together in the mess hall then in "Alter Ego", when he flirted again with B’Elanna at the luau. As for Torres, I guess that her interest for Paris began around "Blood Fever" (the choice of a sexual partner does not take place without feeling a minimal attraction - physical or emotional - with the other individual, right?!). Then, there was "Real Life", where Paris & Torres openly flirted with each other over Women Warriors at the River of Blood, one of B’Elanna’s Klingon romance novel. In "Worst Case Scenario", they talked about a lunch date. And of course, in "Once Upon a Time", they kissed while outside a shuttle while in space.
My problem with this couple is that I have never felt/seen any chemistry. Alright, both clearly shared some
notable commonalities such as being astranged from parents for different reasons and considered as outcasts from their relevant societies but above that, ... . Plus, as much as Paris and Torres were interesting as individuals... even if B'elena's explosive and unmanageable character carried me quickly on the nerves, once together, they lost a lot of what made their charms. And as a married couple, they got quickly boring!
As for Chakotay/Seven, I think the relationship was a "creative" decision taken during the writing of Endgame because 1) it was necessary to give something to do to Robert Beltran, even a futile occupation like the storu of a Chakotay's flirting with Seven, as seeing as the roles of First Officer & close confident were already occupied by Admiral Janeway! 2) and maybe the most important, the decision was essentially taken because Kate Mulgrew refused until the last minute of the show, to see her character being romantically associated to Chakotay -> IF it had been otherwise, that the lead actress had suddenly announced to producers to have changed her mind and finally been ready to give to J/C shippers what they wanted (from the beginning), I don't doubt that the writers would have found a solution to reunite them on screen, even at the very end... or at least, an unequivocal sign allowing the viewers to think/know that the field was still open for an eventual reunion in a very near future. In this case, Chakotay/Seven flirt would have been presented as a mistake from a frustrated Chakotay in need to be loved or just Seven's failing experience to move from a virtual reality with a holo-Chakotay's character to a real experience with the true Chakotay.
-> if producers/writers "were able to offer" us a romance out of nowhere between Chakotay/Seven who barely exchanged ten lines in 3 seasons (s4 to s6) then 2 episodes in s7 (Natural Law then Human Error*) to establish the beginnings of a possible love story in the making before Endgame where they dated
secretly for real, well I guess that they could have done the same with Janeway/Chakotay, who flirted openly and heavily during 3 seasons then, then maintaining a purely platonnic relationship while remaining closey during the 4 following years, don't you think?!
*Plus, I think that as much "Natural Law" introduced us to the idea that Chakotay and Seven
had finally managed to find a base to a cordial understanding and to work together
, when there were only an atmosphere of mistrust, frustration and disdain between them, while "Human Error" (7x16) introduced us to the idea of Seven's will to try to experience what it would have been like to be more human like attending and participating to social events with a simulation of the upcoming baby shower for Paris/Torres & being (more) emotional, like how to react if the individual she would have hoped to become, developped a loving feeling for someone else. Besides, a great part of the story was introduced as an experience. So, it would have been easy to erase the eventuality of a romance betwen them from our minds to offer another story, more persuasive and effective. IF Branoon Braga's idea to kill off Seven character in Endgame was retained, the C/7 romantic scenes being in "Endgame - part 1", would have been irrelevant -> I don't think that to the point where they were in their romantic relationship, Seven would have sacrificed herself to save Chakotay if the latter had been in danger. By cons, I think she would have done it to save Cpt. Janeway or to save her new Collective which was the crew in general.
In the end, only a few was satisfed with C/7's storyline and still, while a great majority remained upset and frustrated. Even the new fans continue to focus on J/C (although personally, I like a single and strong willed Janeway) and will be disappointed to find out that Chakotay will end dating Seven at the end...