Summons, Sage Mode, and Ascension
I've been thinking a lot about summons lately -- where they come from, when these contracts originated, why a summon would agree to help a human in the first place, that kind of thing. Here's what I think I've settled on.
In the same way that any regular fox can become a kitsune if it lives long enough and gathers enough power to grow another tail, every mundane animal has the potential to become something more -- first a yokai/spirit, and then a sage. Most animals won't have the ambition to do so, and many that have the ambition will lack the aptitude, but the potential is still absolutely there. The difference between mundane animals, spirit animals, and sage animals is basically the difference between civilians, shinobi, and sages. This is a "humans are normal-type pokemon" situation.
(Of course, in the same way that a Genin and a Kage are as apples to oranges as you can possibly get, not all spirit animals are created equal)
An animal contracted with a ninja will have a much easier time gathering chakra and gaining power and knowlege than they would on their own, which is really the core of most mundane summoning contracts. An animal with ambition can join forces with human shinobi in order to become stronger, usually with the eventual goal of gaining access to the spirit world. This was, for example, the case with Kakashi's ninken. They're all normal dogs he adopted from shelters, who he's trained to be his shinobi partners and allies in exchange for giving them a better chance at someday ascending. In the case of situations like the Inuzuka dogs, those alliances are passed down generationally. A clan of humans and a clan of dogs working together to someday help those dogs descendants become powerful yokai.
Most contracts probably come with some kind of "don't fuck me over" clause that says the summon can't just dump their human once they reach spirit or stage status, which is the origin of things like the Toad Sage contract. Some ancient human made a contract with Fukasaku back when he was just a normal ass toad, with the agreement that even after he became a sage, Fukasaku would continue to provide support and assistance to the human's children and descendants. Fukasaku agreed because that meant his own children and descendants would have the same opportunities he did -- to train alongside powerful humans and gain power of their own. Eventually the human's bloodline ran out, and the Toad Sage contract became something passed down from master to student.
By the time Jiraiya reverse-summoned himself to the mountain, the Toads had collectively gained enough power that they no longer need human assistance to maintain themselves. The child of a yokai is usually already a yokai and they'll just get stronger as the bloodline continues to cultivate. The elders can train their own young and gather nature chakra from the mountain, allowing them to maintain their status as spirits and sages, but honoring the old contracts is still an important part of their culture, so they agreed to take Jiraiya on, and through him the cycle began again with Minato and then Naruto.
I also imagine a human summoner is expected to provide other resources and materials to their summons when necessary. Manda demands "one hundred corpses" from Orochimaru, presumably to eat, and this was just done to make them seem ~oooooh so evil~ but I do actually think that makes perfect sense. That's basically how domestication works, and while summon animals aren't really domesticated the basic principals of how humans make friends with not-humans carries over. If you team up with me, you won't need to spend so much time hunting or foraging for your own food, and you won't need to worry about a lean year. I will make sure you never go hungry again. With that in mind, summons expecting to have their needs met by their summoner so that they can focus on gathering chakra and power makes perfect sense!
In the case of established, already powerful yokai like the toads, snakes, and slugs, who presumably don't usually need humans to feed them or provide them with shelter, the expectations would expand to include things like resources or materials that the spirits want but would be a hassle for them to get on their own. This can further become kind of a trade agreement, the spirit getting their human materials from the spirit realm and the human getting their spirit materials from the human realm, so that neither of them have to try and brave each others markets.
I imagine gaining a contract with an already-established sage or spirit animal who doesn't have some deep, cultural value for upholding those contracts, is a lot harder. Maybe because the yokai achieved spirit status on their own without help, or maybe because their old contracted human was an asshole. Maybe because the yokai themself is an asshole and doesn't work well with others. Either way, in situations like that it's probably a case-by-case basis, where getting a contract in the first place requires first convincing the yokai to work with you, presumably by figuring out something they want and offering it to them. Then the contract would be based more around you continuing to provide that specific thing than just the usual helping each other become stronger situation.
