Well, as well as being a British citizen … and a New Zealand citizen … I'm now an Irish citizen as well.
OK, I know I'm a bit unusual – in many ways :-). I was born of an English mother and a New Zealand father in the UK at the moment.
My place of birth and my mother's nationality gave me automatic UK citizenship. That's, if you like, my primary citizenship as I live and work in the UK. As a side note, did you know that Britain is the only country in the world that has different classes of nationality? A remanent of having a globe spanning Empire :-)
New Zealand Nationality
However my father was from New Zealand and, although he married my mother and settled in the UK with her, he never gave up his New Zealand citizenship. He did this so myself and my brothers would have the option to claim New Zealand citizenship if we wanted to. It was part of his inheritance to us. Unfortunately he died when I was quite young, but when I reached adulthood, myself and my brothers decided to take up our New Zealand citizenship. Partly because it's always very useful to have a second nationality, but mostly because our father had specifically wanted to give us this option. It was a way of honouring his memory and keeping touch with our cultural roots.
I've since visited New Zealand, using my New Zealand passport and thoroughly enjoyed it. It's a truly beautiful country and the people are incredibly welcoming. I would love to visit New Zealand more often, but unfortunately it is half a world away and at the moment my partner dislikes flying :(
I do like to think my brothers and I absorbed some of the "can do" attitude that marks Kiwis from our father.
I said in the opening paragraph that I was born in the UK. This is completely true. However, I was born in a very special part of the U.K. I was born in Northern Ireland and lived there for a nearly decade. It is this that allows me to claim Irish citizenship as well as British. I have visited Ireland many, many times since leaving and it is like coming home. There are no people as friendly and as hospitable as the Irish :-)
A side effect of being a citizen of both Ireland and Britain is that I'm also an EU citizen. Despite the Breixt vote earlier this year, I will remain so if Britain leaves the EU by virtue of my Irish passport. EU citizenship gives you the right to travel and work anywhere in the EU.
Dual (or Triple) Nationality - the pitfalls
You do have to be careful with dual nationality. Not all countries allow you to have citizenship of another country and if you take another, you will automatically lose the other. I'm very fortunate in my case. The U.K., Ireland and New Zealand are all fine with you holding citizenship of other countries. All have provisions that you can not use your citizenship of another country to avoid your obligations and duties to their country. Personally I would say that if you want to do that, you shouldn't be a citizen of that country. I'm proud of all the countries I'm a citizen of and happy to discharge any and all duties required.
Citizen of the World?
As I said at the start, a fortuitous set of circumstances allowed me to have citizenship of 3 countries. So what does this make me? Well classically a bit of a mongrel. Part British, part Kiwi and part Irish. Is this a bad thing? Definitely not, especially if I manage to snag the best bits of all of them. The stoicism of the British, the friendliness of the Irish and the self-reliance of the Kiwis.
But I like to think that it makes me a citizen of the world :-)