My partner wants to worship the Norse deities but runs into a lot of media that says that he only wants to appropriate them because he (as a white person) doesn’t have any connection to gods of his own. Do you believe you need to be descended from a certain group (in this case the Norse) in order to worship their deities?
Pre-Christian Norse culture/religion was never closed, and especially given how far and wide they traveled (and intermarried), it’s pretty ridiculous to even claim it. As far as I know, the only people who say such things are ultra-folkish and/or white supremacist groups who are seriously misrepresenting things to cater to their own exclusionary agendas.
I would point to this article published in the Iceland Magazine in May 2017, which focuses on Ásatrú (which is currently an official religion in Iceland), but would apply to the Norse Gods as well, and the following quote in particular:
Anyone can practice the religion but only Icelandic residents can join Ásatrúarfélagið
Only Icelandic citizens or people who have a domicile in Iceland can become members of the Ásatrúarfélag, but anyone can practice Ásatrú, regardless of their nationality or residence. It costs nothing to join and is open to all, irrespective of race, cultural background, gender or sexual orientation.
Ásatrúarfélag is the national pagan association in Iceland – which is why that particular group is only open to those who live in that county. But the religion itself and worship of the Gods is open to all. And I mean, if anyone would know, it’s them right?
Now, I do feel that it’s worth mentioning the following though. Whether or not other religions are able to be practiced by everyone depends on the culture/religion in question. Some are open to everyone – Norse and Hellenic practices for example, while others, such as Native American religions, are closed to those who are not members of the culture (or who have not been adopted into the culture). So it’s always better to ask, if one is unsure.
The Informed Pagan 
Actually, if I recall correctly… Ásatrúarfélag closed itself off to Icelandic residents only because back when they opened they received death threats from folkish/nazi heathens for being universalist. That context is super important.
The membership in the organization itself is limited to Icelandic residents, but they specifically state that the religion is open to anyone – regardless of where they live (or their race, gender, cultural background, etc…).
Personally I see universalism in pagan religions as a sort of cultural appropriation: a sense of entitlement to something that has little to do with you, your ancestors, or the culture you were born into. And, as the Sikhs would say, entitlement is a symptom of vanity or hubris – that is, to think you deserve something you lack…
It doesn’t seem germanic peoples were more open or closed than any of their neighbors TBH, and even that means little, given Jews and Arabs notoriously traveled far and wide and were (are) very possessive of their culture.
I think the only pagan religions with a real claim to universalism were Roman and Neo-Platonic Hellenism, but even then, that was more of a facet of imperialism for the former, and a late reaction after it started to die out for the later.
You can think whatever you like, it doesn’t make it true. There is plenty of evidence that old Norse culture/religion was always open, and in that case, as long as one is being respectful in one’s practices, then there is absolutely no vanity or hubris to follow a path one has been called to, regardless of one’s own personal culture/ancestors. Not to mention, it’s not so much that the Norse traveled, it’s that when they traveled, they intermarried and shared their culture and religion with others when they did so, as opposed to others that kept such things separate no matter where they went.
As to other cultures, it definitely depends. For example, even though I personally have a fair amount of Irish ancestry, I would never claim to be a Druid – given that is something, very specifically, that was closed to outsiders and those who were not born/raised within the culture.