I see it as a logical progression throughout Spock's life. The conflicted half-breed with 2 sides warring logical and emotional urges. When the Romulan commander from "The Enterprise Incident" learned of Spock's dual heritage, she played up the similarity to the Romulans' code of honor/duty and their ability to still embrace they emotional side. When speaking of Romulan women she said "as a Vulcan you would study it, as a human you would find ways to appreciate it."
When Spock finally accepted his emotional side for what it was in STTMP, he didn't go all emotional and go cuckoo like Daffy Duck. He began to explore his emotional side but still under the watchful eye of logic to guide those emotions. Once he trained himself, he went on to disciple others like the half Vulcan/Romulan Saavik. He could explore the problems of resolving her conflicting sides and watch them resolve on a personal level. He also discipled Valeris (presumably all Vulcan) with whatever issues she had. Sometimes you have a win like Saavik and sometimes you have a miss like Valeris. But Spock was learning from it.
By ST VI, he was starting to follow in the footsteps of his ambassador father. He helped to reach out and negotiate for a peace with the Klingon Empire at Sarek's request. Apparently, later he and Sarek had a disagreement during their attempts at easing Cardassian tensions. Perhaps Spock by this time was leaning more towards a more liberal emotional approach whereas Sarek wanted to pursue a more conservative logical approach. In both cases Spock had taken his discipleship on the personal level and was going to the next step and started mediating between two nations/worlds.
So it should come as no big surprise, when he finally up and decides to leave for Romulus and start his reunification attempts with Romulus. I don't see it as far-fetched. The Romulan Commander from "Balance of Terror" upheld his code of honor and duty and followed the Praetor's orders but came right out to the Centurion and said that he regretted that their victory would give Romulus the present of another war--one with enough blood to satisfy the Praetor's tastes. He sounded like one who wished for a little more restrained logical reasoning to temper the Preator's more bloodthirsty tendencies. He even told Kirk that in another reality that he could call him a friend. Even the Commander from "Enterprise Incident" wanted to follow her duty and capture the Enterprise but was reasonable towards the Enterprise crew and said she wouldn't hold them responsible for being a good soldier, doing their duty, and following the captain's unsanctioned orders. Unfortunately from The Next Generation and onward the writers didn't understand the Romulans and they degenerated into these "clever," scheming, smarmy bad guys. (The Vulcans didn't fair much better in DS9 or Enterprise .) So I can see Spock going to Romulus and making a connection with sympathizers. Showing them a way to change their empire from within--through the people and by the people-- peacefully in a way he thought Surak would have approved. He saw the Romulans ability to value their emotions as something that could benefit the rigid "Herbert-like" Vulcans. He saw the Vulcan logical discipline as something that could benefit and temper the Romulans' more aggressive soldier mentalities.
