And is what is selfish to one, selfish to another?
No it will never be because we each have different experiences and lives.
Is classing someone as selfish a statement of fact, or are we simply passing judgement on them?
It can be either or both depending on the person. We all pass judgements and sometimes without realising it.
Is being self centred or self obsessed selfish?
To me yes it is but to others there might be a reason why it is not selfish; maybe they've become self-centered or self-obsessed because of what they've had to go through in their life. Maybe they were forced to become so from their upbringing or because of a partner or because of where they work or live.
Or is it just human nature to look after one's own interests and needs
first and foremost?
It can be said that this is human nature but again we have to look at a persons life and experiences before we come to a decision. Some people have had to fight their way through life where others have had it pretty easy. Some people can open their heart and lives to others where as others find it diffcult to open up and trust people.