Someone said that the idea of "Akhand Bharta" or "Undivided India" is a myth and quoted Winston Churchill as saying that India was as much a nation as was Equator. He said that India once was divided into hundreds of small kingdoms and hence the idea of a single nation itself is absurd; forget about the idea of a Greater India with regions like current Afghanistan being part of it.
First, countries do not just become; those are made; often by great people. Every country was once just a geographical area or a collection of scattered villages or tribal territories with nothing in common. So looking at any idea with a baggage of historical account fixated in mind does not help.
It is true that once there was a greater India which was unified for certain periods in history. During the time of Ashoka or during Maratha empire, India was extended on the north. But due to Islamic uprising, Afghanistan region took different path. Even today, the names like "Afghanistan" etc have Sanskrit origin (Sthan = place).
No matter what, as Indians we feel proud of the fact that once we were greater than we are currently. And there is nothing wrong in it.