Birth. And of course it's not an asset because all being born into a particular country results in, is responsibility that you inherit. It's like being born into a tribe, but citizenship goes a step beyond because certain rules must be followed to keep your freedom or whatever you managed to earn in your life.
To pursue citizenship outside of where you are born you have to go through a long and difficult process. Then you in most cases have to renounce the citizenship from where you were born.
I see! The system is always set up to fail its people. Go figure!