Because it pollutes the global namespace with heaps of stuff, making variable name clashes more likely.
It probably won't affect your program but it is good practice to either put std:: before each std thing, or put using statements like this:
1 2 3 4
using std::cout;
using std::cin;
using std::endl;
//similar for all other std things
I do a mixture, the above using statements for things that are used frequently, and std:: for other things that only used a couple of times, like std::find or std::sort say.