Southasia is a term that is now in vogue. There are histories of Southasia, there are journalists' associations that style themselves Southasian, there's SAARC, and every time a Test cricket match between India and Pakistan goes well we're all (temporarily) Southasian. As a region in a physical-geography textbook, Southasia makes sense. There's the monsoon that waters most of it, the great dust cloud that pollutes all of it, the mountains and seas that give it plausible boundaries. But if you trade in the physical map, all greens and browns, for a political map filled in with bright primary colours, if you consider Southasia as the idea that underwrites SAARC, it is hard to know what it really means.
Southasia consists of India and a bunch of countries that share a boundary (land or sea) with India, but not with each other (except for the late entrant to SAARC, Afghanistan). India defines Southasia, not only because it is by far the largest country, but also because the others are connected to one another at one remove, via India. Southasia feels unified when Punjabis cross the border and exclaim at similarities, or when Bengalis from either side of India's eastern borders do the same, or when Sri Lankan Tamils like Muralidharan come to find brides in Madras. It is India's diversity that gives Southasia meaning. Otherwise, Nepalis don't feel a special kinship for Tamilians, nor do Sindhis feel intimately linked with Sri Lankans.
The irony is that this Southasian identity, this idea of a regional family of nations that would have no meaning without the connections supplied by India, is made up of countries established on principles diametrically opposed to the idea and reality of India. India was founded as a pluralist democracy, and it has remained one for more than fifty years. Pakistan, Bangladesh, Nepal and Sri Lanka, on the other hand, are sporadically democratic, avowedly majoritarian states, owned by dominant religious communities. Pakistan and Bangladesh style themselves as Islamic republics; until recently Nepal was famously the only Hindu state in the world; and Sri Lanka altered its Constitution to give Buddhism the 'foremost' place in the life of the nation. The ruler of Bhutan, rightly or wrongly, is committed to cultural homogeneity, and spends his time policing his 'Nepali' subjects or expelling them. The one non-Indian attempt at pluralism was undivided Pakistan, and the reflexive chauvinism of Southasian identity (always excepting India) put paid to that.
Pluralist versus the rest
To put this difference in terms of nationalism, India's neighbours believe that nations are built to house particular communities, or that the deeds to the nation are properly owned by its majority community. Think of an India where the Bharatiya Janata Party (BJP) has been in power for decades, where Hindutva is formally enshrined in law and the Constitution, where victorious cricket teams take their trophies to be blessed by Sankaracharyas, and you have some idea of where the best of these Southasian states, Sri Lanka, is today. One way of understanding the BJP is to see it as the archetypal Southasian political party, committed to that great Southasian project, the sectarian state. Thus, India would become a properly 'Southasian country', and SAARC would become a 'family of nations'.