Its a matter of control. Who can modify a global? What is the lifetime of the global? What if I want to go parallel? Globals are like cats; they may look cute, but will they do your bidding easily or at all?
If the purpose of a variable is to be continuously modified in many places by the user such as std::cout or std::cin, then making the variable global seems reasonable. Here are some links for more info. For the most part, your variables are probably only going to be managed by a few functions that can easily be wrapped in a class or by passing as parameters.