Proud to be ...insert nationality here...?
This makes as much sense as being proud that the Earth orbits the Sun! Like the location of your birth, you had nothing to do with the relevant processes that brought this state of affairs into being. Both are physical facts no reasonable person could or should take credit for.
Remarkably, society has engineered a gigantic theatrical performance in which many people act as if location of birth is a measure of worth. Unfortunately, most of the players have forgotten that it's just a show, and their ideas about the value of human life translate into real tragedies on every scale imaginable.
They forget that we are all the same species, and that the world they grew up in was not one of their making. As a result we have flag parades, refugees locked up in prison camps and TV programs where people go on and on about how 'our people' have a set of intrinsic qualities that consist exclusively of the best aspects of human behaviour (of course!).
Perhaps the orbit of a planet around a star is simply too far removed from the individual to invoke warm fuzzies. Imagine then if the play changed so that being born on Earth was the metric by which human worth was judged. This would eliminate the whole pride thing - being born on Earth, or even in the Universe, is something everybody has in common. Of course, we don't need an elaborate theatrical conspiracy to exist for this to be true!
Nationalism turns out to be just another one of those ruses used by the unscrupulous to divide the foolish for their own ends. Give a bunch of people a simple truth/identity to zealously latch onto, then label all others the enemy - the result is a slavering mob baying for blood at all costs. This is the extreme to which national pride will run, if let off the leash - a leash usually found in the hands of those who aspire to lead us. Observe as they loosen their grip whenever times get tough.
So if you really must get engage in flag-waving nonsense, fly a flag for Earth. Better yet, fly a flag of you, celebrate your own traditions and conduct your own interpersonal geopolitics. Take the future of your solo nation into your own hands and get teary-eyed as your personal anthem loops endlessly in your head! If you are going to get into ridiculousness, you may as well go all the way! Just keep in mind that you've no more claim on this world than any of the other six billion or so nations swarming on its surface.