It can be better explained in this way also.

Thumb Rule When a copy constructor is called ? : when any object is created and it is being initialized with otherObject of same type Copy constructor  is called. Blv me this is is only one way how copy constructor is called.

Now lets understand one thing with an example .

int foo ( int age) { } and this function is called from main () { int m=10; foo(m); }
now here in function foo(int age) , the varible age is being created and initialiezed with another value which is being send from main with name "m" .
similarly in the case of copyConstructor

class A { ..... A(A&a){}..}
if A(A&a) would be  A(A a) , then here in A(A a) an object a is being created and initialiezed with anoher object . and it will again call copy constructor as per the thumb Rule above. then again same story and this will continue untill the applicationsl stack is not overflowed. and program will fianaly crash.