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.)