Been thinking about this a little bit - the three main men in Kathryn Janeway's life.
First up, Justin Tighe — a scientist, a soldier, and a pilot — who is assertive, fiery, and dominant. He's bold, adventurous, and not so good with people. When Kathryn and Justin meet, it doesn't take long for their relationship to become a burning, passionate affair. They're both very young, both very driven, and they're engaged in next to no time. Unfortunately, the bright spark that was Justin is soon gone — killed in the same accident that claims the life of Edward Janeway and almost kills Kathryn. Learning to live with their deaths is nearly too much for her to bear. Phoebe literally drags Kathryn back to the land of the living, and her old school friend Mark drifts back into her life...
Mark Johnson is a philosopher, and he's safe, inoffensive, kind, polite, and kinda bland, can't lie. Kathryn's relationship with him is not an all-consuming fiery passion. Kathryn and Mark are slow and steady, together for years, separated by Kathryn's assignments, still living apart even after becoming engaged. Mark is a comfort blanket to her — someone dependable to go home to. He's very intelligent, which is good, as it means she's not going home to an intellectual desert (she couldn't be with someone who couldn’t offer her a good debate or argument once in a while).
And then there's Chakotay — a scientist, soldier, and pilot (apparently) — who is assertive and can be fiery and dominant when he needs to be. He was bold and adventurous when he was younger, and probably still likes to get into the odd scrape, but his wilder instincts have been tamed by time and circumstance. What happened to his home colony, and afterwards with the Maquis, taught him a lot. But Chakotay is also a safe place — he's kind and polite. He can do slow and steady if he must (think of all those dinner dates!), but he prefers being passionate. If it had been up to him, he and Kathryn would have had an intense romance from the start. But he understands the pressures she's under and gives her the space, understanding and safety she needs (until Endgame, but he's clearly been forgiven for that transgression... he is after all, a man!).
There are some similarities between the three men, but I don't think things would have worked out with Justin or Mark — even under optimal conditions. With Justin, things were probably always going to be too much. Kathryn was so young when she fell in love with him — only in her early 20s. And the demands of their careers might have pushed them together too soon (maybe eloping between missions), or it could have pushed them apart on different assignments. They were both young hotheads; things probably would have gotten messy.
And Mark... Kathryn needed safety and stability after losing Justin and her father. He was a comforting, familiar face... but was that ever going to be enough? How long had they been engaged? How long would it have taken to get a wedding planned if they couldn't even move in together after so many years as a couple? I think it would have all fizzled out eventually.
But with Chakotay? He's kind of a mix of the two. Not quite as fiery and hotheaded as Justin, but not as slow and steady as Mark. Chakotay's got a similar background to Justin — growing up on a colony world, but choosing to go his own way. Opting for a blue Starfleet uniform to begin with, then showing tactical skills (Justin as a Ranger, Chakotay as a tactical instructor and then a Maquis). And then there's the fact that Chakotay trained as an anthropologist, while Mark trained as a philosopher — two disciplines that are, in many ways, adjacent to one another.
So yeah... I think for Kathryn, Chakotay is in the Goldilocks zone — not too hot, not too cold... he's just right.
And I think him having so much in common with Kathryn's two fiancés is far from a coincidence!
