India is directly bordered by their biggest rival that they have constant conflicts with.
Well India didn't have an event where the locals that spoke different were massacred or reeducated/"civilized" like the other super powers of the US and China.
Actually the idea of modern India is mostly a product of the British, so in that aspect they are a young nation.
To add to this, decolonization of the british raj wasn't all roses and sunshine either. (the split of india and pakistan was especially)