Brain drain can be due to any reason lack of opportunity, salary freezes health hazard, political instability etc.
and even I agree to the fact that every person has dreams and migrating to fulfill their dreams is not wrong. Definitely it’s the country's loss.
But if a particular person performs well outside the country it is a matter of pride for the country itself, because that person may not have got opportunities to perform that better in his own country. And as far as our country is concerned over population, high unemployment are among the major factors leading to brain drain.