Not because of...
His mother from his previous life? But... she had been his only attachment in his previous life. If there was some plan directed at this Naruto World, then the most likely reason would be that one and only attachment from his previous life! Looking at the utterly unchanged expression on the young man's face, Chiba knew that the young man's words contained neither lies nor concealment.
His other self truly had no attachment at all to that mother who had left such a deep, deep impression in his memories.
Or rather, not much attachment.
And honestly, ever since Chiba had developed the thought of returning to his previous life to see his mother, that thin, frail figure had become increasingly clear in his mind. At the same time, he finally understood where that feeling had come from—the feeling, when he returned to the new cabin after being away from the village for so long and saw everything neatly arranged, that someone had once done such things for him too.
He also finally understood why he had grown close to Kushina so quickly, and why he had ultimately decided to save her from the mire of fate at any cost.
Kushina disregarding everything for his sake and risking her life to save him was one reason, of course—an extremely important one.
But clearly, before Chiba had realized that his memories were completely blank, he had poured his profound desire to repay his mother into Kushina. It was not that he regarded Kushina as a mother figure, but rather that the urge to repay her, already branded deep into his heart, had found an outlet. And Kushina of this Naruto World was that outlet.
That was why, even without his memories, he had made up his mind so quickly.