I think the showrunners likely thought that Seven had to be shown as being nearing the end of her individuation process as they were approaching home. Rather than doing this by having her raucously join in on karaoke night on the holodeck or becoming B'Elanna's parrises squares playing best buddy, they chose to emphasize a conventional marker, pursuing romance.
Now, I'm of the opinion that Seven's choice of object didn't really come out of left field. I have the sense that Chakotay made a, well maybe not positive, but firm impression on her going all the way back to Scorpion II, in the cargo bay and the conference room a little later on. Here was someone that was forceful, straightforward, and understood how the alliance would play out from the Borg's perspective a bit more realistically than his CO. Not that Seven appreciated that perspective at the time, of course.
But I think that a resonance of the early encounters may very well have remained with her as time went on. He was solid, substantial, with a strong cast of mind, and, perhaps in line with tropes present in the socialization holo-programs the Doctor was feeding her, he had the requisite dark, hunky good looks. Most importantly, she didn't have the opportunity to have her view of any of these positive attributes dispelled by virtue of never being in the ready room during those tense situation when Janeway made short work of her hero.
Also, who knows what subconscious bond was indelibly forged when they were linked at the end of Scorpion. While left unremarked upon, such an impact may have loomed large down the road.
When it came time to punch her "fully human" ticket by forming a significant attachment, who else could have fit the bill? Okay, okay I know, Harren is the obvious answer. But I just dont think Seven could have maintained respect for him after pointing out that he was going down the wrong track with his cosmological noodlings. The Doctor? Huh. Every now and then I have to get my fill of seeing Seven's expression when he's down on his knee, pouring his heart out in Renaissance Man. Might very well be my favorite screen capture of her face of the entire series, and it says it all!!!
