Democracy fundamentally relies on freedom of expression: being able to discuss problems, to organize protests, to resolve issues (like corrupt politicians).

If you conduct mass surveillance (as does the NSA, the brits and apparently many others too), then people will refrain from expressing themselves freely, out of fear of blowback. Problems will not be discussed anymore, solutions will not be proposed and corruption will flourish. That's the end of democracy.

Usually, schools teach these basics.