Why we should never declare global variables

Answer 1

'Never' is an exaggeration, although global variables do have their cons. One of the most direct reasons is because a global variable is subjected to change everywhere - Functions, class methods, main(). This makes it hard to trace, and your program equally hard to debug when errors occur.

Answer 2

Depends on need.
If you need a global variable, go for it.
Usually, the story begins from main() and ends in main(), that's why people use local variable.

Answer 3

Global variables are typically used in game programming because they are much faster to read/modify.