The Boy and the Beast ★★★★

I really enjoyed it, but I still have some questions about things that didn't completely made sense to me. I'm sure as usual I missed the deep philosophical meaning or whatever.

Who decided that the bear and the boar were the only ones to be able to compete to be the new supreme leader or whatever?
When Ren is a kid, the passage to the human world closes. But when he's a teenager he just comes and goes at will. Why doesn't anyone else just go into the human world?
Oh no, sorry, they do. The bear, once, when he decides for no reason to take this random kid as his apprentice, and once when the boar finds an abandoned infant. Why were they in the human world if they fear humans so much?