What's ultimately at play here is the boundary between what an individual can expect to gain from their community, vs what a community expects from its individual members.

The reason why nationalism and patriotism exist and persist, and are often celebrated, is the same reason why loyalty to one's tribe or family exist and are often celebrated.

And the reason they are all celebrated is because the fates of individuals are often bound up in their communities. And so to some extent, it often does make sense to demonstrate a certain amount of loyalty to your communities - including your family, your tribe, or your country.

Although of course, ideally, there is a healthy balance to be struck, whereby the individual community members can all gain from their collaborative relationship, but where the community doesn't ask more value from any individuals than what the individuals receive in turn. And most people will recognize that loyalty "until death" is almost always an instance of community overreach.

So in that light, JFK was obviously wrong when he said, "Ask not what your country can do for you – ask what you can do for your country". Such a precedent, if taken literally and absolutely, would clearly constitute community overreach.

But the other thing to recognize is that there absolutely is selective pressure that causes communities to demand a certain amount of loyalty from its members. If there's no loyalty at all, than there is no community.

I think its obvious that the ultimate goal, which would be best for everyone, is to form a single global human community, as you say, where nationalism wouldn't be a thing anymore. But the problem we're in now, is that there are game theory dynamics, as well as cultural ones, that make it difficult for all of the communities of the world to simultaneously abandon their own claims to sovereignty in favor of forming a single global community.