Like Huxley said, it's about the why.
An NFP will generally be most self-sacrificing in situations where they feel the sacrifice is representative of, and highly meaningful to, a greater cause. Lady X expressed it right - sometimes the cause is more important than oneself. That cause can be anything from ending injustice in the workplace to fostering a good romantic relationship.
The Great One said:
But, I would think that Fi is more about what "they want" and Fe is more about what "others want" and it seems like it would be way more of an Fe thing than an Fi thing?
This is a pretty simplistic definition of the Fi/Fe split - more accurate is to say that Fi looks internally first and then moves outward, while Fe looks externally first and then moves inwards. This results in Fi users being more aware of their personal feelings and emotions, but also attentive to others' internal feelings, as well as the feelings
within groups. Fe users, on the other hand, are more aware of feelings and relationships
between themselves and others, as well as between members of groups, and between groups themselves.
So it's not that FPs are more self-centered, it's just that we tend to look inward with our F function just as TPs look inward at systems with their T. And we do look inward to our values to see if something is worth sacrificing our time, money, effort, or feelings for. INFPs in particular are generally recognized to be quite self-sacrificing.
cascadeco said:
Si is another powerful element that comes into play in embracing and maintaining cultural traditions and principles
I think we need to be careful about Si, because it's frequently misunderstood... as far as I understand it, it's more about drawing wisdom from past experience than about maintenance (lol, maybe in ISxJs it often manifests like that saying about "if it's not broke, don't fix it")... I think it's almost more of an NF thing to romanticize self-sacrifice, as well as cultural traditions... in general, I suspect most SJs will self-sacrifice if they believe that is the most reliable method to achieve their goal. I think that's true for both my ESFJ mom and boyfriend.