Just as we humans aren't all the same, and in the face of difficulty we behave in different ways, so some materials, like felt and leather, improve if you hammer them, and so does iron, which, if you hammer it, spits out the dross and becomes stronger...
You have to be careful with similes, because they may be poetic, but they don't prove much, so you have to watch your step when drawing educational or edifying lessons from them.
Should the educator take as his model the smith, who roughly pounds the iron and gives it shape and nobility, or the vintner, who achieves the same result with wine, separating himself from it and shutting it in the darkness of a cellar? Is it better for the mother to imitate the pelican, who plucks out her feathers, stripping herself, to make the nest for her little ones soft, or the bear, who urges her cubs to climb to the top of the fir tree and then abandons them there, going off without a backward glance? Is quenching a better didactic system than the tempering that follows it?
Beware of analogies: for millennia they corrupted medicine, and it may be their fault that today's pedagogical systems are so numerous, and that after three thousand years of argument we still don't actually know which is best.