I've never lived in Belgium but was born there, speak the language and care about its future - why should I not be able to vote there?
Because your choices won't have personal consequences. In the case of Belgium, no big problem. But let's take the death penalty as the best example for my argument.
How can someone vote for the death penalty when he's not affected by that law. Not fair.
Voting for a rain-tax in Britain is easy when you live in the Sahara.