Just how much resistance do we need?

Writing and reflecting about my study of children’s interplay with 3D materials, I get more and more astonished how important experiencing resistance seems to be for learning. We have all experienced (and learned) that some kinds of resistance are important in order to learn, get self-confidence, strength and motivation. But pushing someone too far beyond hers/his “Zone of proximal development” can in many ways be damaging. How can we know where someone’s Zone begins, and where it ends?
Without connection to my study, my son and I decided to get a budgie. When we went to a pet-shop we were told to keep only one bird if we wanted it to care about people and eventually learn to talk. So we bought only one. But we also bought a book that said something like: Never keep a budgie alone. Another bird is as important to it as the air it breaths (Birmelin). “Who should we trust, the book or the shop assistant” my son (12) asked? How would we know who was right? We had to get our own experience.

The little, green bird was very tame and quite. We let him fly around and gave him a variety of food. But he looked sad... My son and I discussed what we should do: Was it more important for us that he one day learned to speak, or that he was happy? How miserable did we have to make his life in order to get little amusement for ourselves? Just how far were we willing to push, press and stretch his Zone of proximal development? (Sorry for my stretching of Vygotsky’s term in applying it to the non-human world.)
The bird kept sitting on the same branch and did not eat. Was our amusement so important that we would let the bird pay with his life? “It was not!” – we agreed and bought our budgie a nice young friend. They might thank us for that in their bird-language, but who knows, one day they might still speak in duet in human language -?

Birmelin, Immanuel: Undulater, Cappelens kjæledyrbøker