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I am staunchly opposed to national borders. I don't think it's right to discriminate against people for where they are born.National borders are not about "discriminating against people for where they are born", anymore than your family house discriminates against people based on which family they were born into.
One would assume you don't just let anyone in your house, or feed everybody just like you do your children, or share your family assets with the whole neighborhood (even if it would be nice if you did, it, for one, would live less for you. Plus, not everybody is nice and altruistic, but most manage to be that towards their family).
People build a nation as a community, often fighting hard for it to exist (e.g. wars of independence), working hard to make it better, building it's culture, and so on. They have a right to want to preserve their preferred way of life, their culture, and the country/infrastructure/etc they have built (of course not everything is always harmonious inside a nation, the same way things are not all roses all the time inside a family. No human made thing is perfect - and no natural made thing, for that matter).
If nations did not exist, people should still have the right to invent them, that is, to create their own communities with people they share their living on a place, hardships and culture with, build their own thing, and protect it against people who don't share their values or haven't contributed to building them.
In fact nation states didn't exist in the far past, they are a development of enlightenment and modernity. We had empires, and feudal fiefdoms loyal to some remote king, and such before, and people literally struggled to break their pre-nation-state nations (ethnic, cultural and historical groups) free from them...