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Aside from Gamamaru, the true Toad Sage who had existed since the age of the Sage of Six Paths, the two Toad Sages now perched on Jiraiya's shoulders, Fukasaku and Shima, were the oldest and most trusted by the Great Toad Sage in Mount Myōboku. For Jiraiya to enter Sage Mode, he needed the help of these two Toad Sages. That wasn't to say he couldn't enter Sage Mode without them; it was simply less stable. More importantly, with the two Toad Sages there, Jiraiya could skip the step of absorbing Natural Energy and converting it into Senjutsu Chakra himself.
When Naruto went to Mount Myōboku to train in Sage Mode in the original story,
he followed the same template as Jiraiya.
As long as Naruto completed the initial Sage Mode training,
sensed Natural Energy, and learned to draw it in and contain it,
Fukasaku and Shima could then help him complete his Sage Mode training.
Later, because Naruto hadn't yet reconciled with the Nine-Tails,
Fukasaku and Shima couldn't truly fuse with him and absorb Senjutsu Chakra on his behalf. That forced Naruto to undergo a more advanced form of senjutsu training than Jiraiya, in the hope that he could master the perfect Sage Mode and convert the Natural Energy around him into Senjutsu Chakra by absorbing and refining it himself.
Each approach had its own advantages
and disadvantages.