Many children have a prodigious talent for copying. Memorizing jokes, stories, sometimes entire movies. Copying others makes sense – you're barely a person, and everyone you see is more interesting, funnier, more brave, more caring. You should copy them.
As you grow older, you find your own voice, and begin feeling some disgust and shame at the complete copying of others. This is natural – it's as though you're saying even the adolescent you is so inferior as to still be built of copies. It is a shameful thing. And those feelings help us come of age, and build who we are, faster.
Of course, if you'd like to learn a language, copying people is utterly essential. The sheer volume of copying and memorization is immense and dwarfs any other human endeavor I can think of – certainly my own trade skills involve a great deal of understanding, but the level of memorization is completely manageable.
Perhaps this is the cause of language becoming more difficult to acquire as you grow older? Obviously, this is pure speculation, and should be treated as no more certain than a guess, but I find myself making guesses about the world with it, which is the mark of a good theory.