There are two words that summarize how quickly love can develop: "hero saves the girl" and "shared hardships."
I believe that if these two go through all these experiences together, their feelings will develop very quickly.
At first, he did this because Tsunade is Hashirama's granddaughter, so he saw meaning in it. But over time, he realized that Tsunade's special status made her the perfect person to push Tōshirō toward deviation, and that's why she became the only option.
If the plan goes as expected, the two will likely fall in love. After two or three years, this love will deepen and mature.
When Tōshirō experiences the most beautiful thing in the world, Madara will intervene and tear it all apart with his own hands — by killing Tsunade.
He will do this right in front of Tōshirō, and the reason will be betrayal between the villages.
At that moment, Tōshirō's heart will shatter completely, and the shock will cause a large amount of special chakra to flow into his eyes, which will contribute to awakening the Mangekyō Sharingan, plunging Tōshirō into darkness and hatred for the world.
Of course, to ensure the success of the plan, Madara placed a psychological suggestion jutsu within Tōshirō throughout all these years.
Psychological jutsu usually has time-related flaws. For instance, it could make Tsunade fall in love with Tōshirō, but that suggestion might wear off the next day, and Tsunade would return to her normal self.
High-level psychological jutsu requires extremely complex conditions, and the most important of these conditions is that the person under the jutsu must be in an emotionally unstable state. The more intense the emotions, the stronger the jutsu's effect. This way, the jutsu becomes more effective.
This jutsu was a form of insurance that Madara placed within Tōshirō.
The awakening of the Mangekyō Sharingan would be the perfect moment to fulfill his plan.
(Author's note: Adding some details to better explain Obito's character transformation, because it's unreasonable for a great leader to change after hearing just a few words from Naruto.)
Once Tōshirō sinks into the power of the Mangekyō Sharingan, he will begin to lose his sight. At this time, the clues left by Madara on purpose will come to light.
After learning that Madara's Mangekyō Sharingan doesn't cause blindness, Tōshirō will seek a way to restore his vision.
At that point, Tōshirō will find Madara's eyes through the clues the latter left behind, exchange them for his own, and then discover the truth behind the Infinite Tsukuyomi Plan.
At this point, Tōshirō will become the true heir to Madara's unshakable will.
Of course, the plan is just a plan, and things may not go as expected, but this is the general vision.
The first step is to find an opportunity to bring these two together.
So, when he heard Tōshirō express his unwillingness to carry out the two missions, Madara, controlling Itachi, intervened with a serious face: "You can't back out now. These missions are vital for the future of the clan, and you must carry them out."
Tōshirō, frustrated, shouted: "No! I just want to take on simple tasks like taking care of pets and live a peaceful life. I just want to get married and live an ordinary life! Why can't the clan leave me alone?"
"Tōshirō, you're grown up now," said Elder Itachi in a guiding tone. "Stop thinking about what the clan can do for you, and start thinking about what you can do for the clan."
"Hmm..."
"Not to mention the benefits you've gained from being part of the Uchiha clan, but your actions over the past few years have seriously damaged the clan's reputation. The clan members are unhappy with your behavior and feel that I've been too lenient with you. I bear a lot of pressure every day because of you, and you should consider this old man a little."
Tōshirō, irritated, said: "But..."
Tōshirō had received many benefits from Elder Itachi, so he had to go despite his reluctance, but he decided to grumble a bit to maintain his character.
Elder Itachi closed his eyes, sighed deeply, and said to himself: "There's no other way."
Tōshirō was surprised.
Elder Itachi turned and said, "Come with me."
"Where to?"
"We're going to fight."
"What?" Tōshirō was startled and said, "Elder Itachi, are you planning to use force to subdue me?"
Then he glanced at the elder, who was leaning on a cane, his face full of wrinkles and his body frail, and couldn't help but tease: "Elder Itachi, no need for that. You've gotten old, and if you hurt your back, I'd be the guilty one."
"Are you underestimating me?"
Elder Itachi's eyes sharpened like blades, and Tōshirō felt immense pressure as if he were being pierced by a sword. He took a deep breath, then exhaled and said jokingly: "I was just joking, don't take it seriously."
Despite his age, his aura was still formidable. No wonder he was one of the warriors who survived the Warring States era.
"Let's go."
Tōshirō nodded and silently followed the elder. After a short while, they reached a small river that flowed around the Hokage Rock and out of the village.
The place was completely isolated, and no one usually came here.
Elder Itachi gripped his cane with both hands and struck it against the ground twice, signaling Tōshirō to make the first move.
Tōshirō didn't hesitate and immediately used the Water Release: Water Fang Bullet technique to test his abilities.
Elder Itachi's abilities were mysterious, so Tōshirō didn't hold back any of his power.
The river water responded to Tōshirō's jutsu, forming several whirlpools and shooting out four water pillars towards the target.
Elder Itachi smiled, taking advantage of the surroundings to launch a counterattack, noting that Tōshirō knew well how to utilize the environment in battle...
But that was as far as it went. Elder Itachi opened his eyes, revealing his three-tomoe Sharingan, and engaged in battle.
He leaped to dodge the water attacks coming from four different directions and narrowly avoided the hammer blow that Tōshirō was preparing with his raised hands.
Though his attack failed, Tōshirō wasn't discouraged. Instead, he quickly changed hand seals and placed his palms on the water surface left by the missed Water Fang Bullet.
"Ice Release: Sword of the Frozen Peak."
In an instant, a massive ice pillar shot up from the water surface toward the sky.
Water Release can use nearby water to boost power and reduce chakra consumption, and similarly, Ice Release can use nearby water to achieve the same effect.
The timing of the technique was perfect, as Elder Itachi was mid-air, making it difficult for him to dodge the attack.
Elder Itachi squinted slightly and let out a soft hum: "Not bad."
As he spoke, his hands had already formed the seals for a new technique: "Fire Release: Great Fireball Jutsu."
A massive fireball collided with the ice pillar, instantly turning it into a dense white mist.
Tōshirō glanced at the elder, who landed gracefully, and said with a hint of mockery: "Of course, it wasn't that easy."
At that moment, Elder Itachi spoke: "Tōshirō, show me all your power."
Tōshirō, frustrated, replied: "I've already shown you all my power."
"No, I mean activate your Sharingan."
Upon hearing this, Tōshirō's eyes widened in shock.
(End of Chapter)
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