And the problem with languages in particular is that they're taught the opposite of how people usually pick them up. Kids learn the entire language before they ever hear of "grammar" or what it does. They learn the language in words and phrases, and when certain regularities start popping up, they apply those regularities to new sentences and new words. It comes naturally. And when they actually start learning grammar in school, they have a huge base of examples to illustrate what every rule means and how it's applied. "Oh, so that's what I've been doing all this time, without ever having to think about it."
The way foreign languages are taught in schools, on the other hand: you learn the rules first, and then try to learn the language around the rules instead of letting you pick them up as they pop up. And when you try to form a sentence, you have to take every word, translate it to the other language, run it through all the declensions, conjugations or whatever mutations they require, and then shove it into place. Manually. Repeat with every other word in the sentence. Hope for the best. Choke. Start hating the language. Repeat. It's the equivalent of that "you are now breathing manually" mindfuck. You get the yips and get stuck at the most basic of actions that used to come naturally. Only you can't take a break, relax and let it come back to you, because you haven't been taught enough of the language for these things to come naturally.
FFFFFFUUUUUUUUUUUUUU...
And most importantly, language is taught through conversation, not through learning a bunch of grammar by heart and then repeating it in front of a blackboard like a parrot.