Initially, it really frustrated me that the Borrowers and (friendly) humans don't try to talk or work things out (especially after Sho helps Arrietty save her mother). It just seemed to me that there was so much a human ally would be able to do for Arrietty and her family... and, for that matter, an awful lot that Borrowers could do for humans. Just think how much of a difference a tiny-sized, intelligent worker could make in fields requiring small-scale precision... like, say, heart surgery.
But then it occurred to me that that's the entire point. The film is intended to be a bittersweet tragedy. In spite of good will on both sides — think of Sho's grandfather and his dollhouse — and the real possibility of a better future, humans and Borrowers are inevitably separated by opposing cultures and attitudes. Haru's unable to view the Borrowers as anything other than thieves or household pests, while Arriety's parents are too bound up by their traditional (and perhaps entirely reasonable) fear of humans to consider that they might be missing out on a better way of life (Arriety's mother, for instance, is so scared of Sho in the scene where Arriety introduces them, she can't even look at him). Ultimately, in spite of their bond, Arriety and Sho are pulled apart by their families/associates' inability to coexist with one another. Kind of like Romeo and Juliet, only it's more about friendship rather than love (Ship Tease aside) and nobody dies.
It also continues a running theme in Ghibli's work of good people ultimately being separated by cultural divides (see Princess Mononoke, for instance.)
Personally, I reflected upon the fact that Borrowers and humans going their separate ways was how it had to be, simply because there cannot be an equal relationship between the two races as humans are always going to be holding all of the cards... No matter the situation or interaction between them, it would always be the Borrowers who would have to entrust their lives to the humans' mercy and good-will: A human offering to gift them something? The Borrowers have to trust that the human is not setting a trap for them. A human seeing a Borrower? The more the humans know about the Borrowers, (how many they are, where in the house they live, just the fact that they exist,) the more said knowledge unmakes the only leverage and source of ensured safety that the Borrowers have in this unequal relationship: mankind's ignorance. Arrietty allowing Sho to carry her? Is both an example of her trust in him, but also an example of her desperation in face of the danger Homily was currently in... It's not a question of culture or tradition keeping humans and Borrowers apart, it's a simple question of the Borrowers not having to put the safety of their lives in someone else's hands, and mankind's inability or unwillingness to give up their own privileges and position of power for the sake of giving the Borrowers' the leverage and safety they need.
