Does this even work, though? Does "if I were them" mean "if I were experiencing the same type of situation they are"? In my experience, how I feel in situations is often quite different from how others feel, or at least how they say they feel. This is why I hesitate to make assumptions about other people's reactions and behavior based on my own.
I think with Fi dom/aux it's just "instinctive" with us and we don't even consciously think "if I were them" ...it's not even that contrived. We just feel pain for another's suffering. As Fi types grow older, though, I think it does help to develop other functions so that this empathy is as accurate as possible. I think Ni development has really helped me to look at things from other perspectives.
It's weird, that sometimes I can FEEL that other people have Fi, which leads me to my next point...
What reason - because we have only tertiary Fi and aren't that good with it?
Um a lot of times IxTJs can seem self-centered and wayyy too guarded to a much older age because their Fi is tertiary rather than dom/aux, which can be frustrating of course for someone who strongly values Fi. I say that INTJs without Fi development are some of the most infuriating people...but that's not even true. IxTJs always have this little soft spot, even when they're totally immature, so they just seem kind of like a self-congratulatory self-absorbed kid...sometimes when they're 26. I mean xxFPs can be this way as adults, too, but Fi tends to get more and more ethically refined with age and maturity, and this happens more easily in the dom/aux position.
I feel that IxTJs are very sensitive when I'm close to them, though. Like if they actually let you in at all...I swear it's like I feel it. I remember the strong, strong awareness I had of it in my grandfather as a child, like I felt like I understood his inner person, as much as his inner person could be vaguely understood, because he was a very private, guarded individual who tended to show love by deep loyalty, consistency and acts of service, as well as occasional displays of lavish generosity rather than being very "emo."
I knew JTG was an ISTJ for a similar reason. Like he communicates his feelings through music, and told me about himself, and I was like OMG YOU HAVE Fi. And at first it was just a feeling, like an inner knowing, and later he actually described to me verbally manifestations of tertiary Fi in an IxTJ.
However, with strange IxTJs (especially young-ish INTJs) sometimes I have to remind myself that if they are acting bratty, they are actually squishy on the inside.
I think mirrors can frustrate people. Like "your immature Fi reminds me of my bad/immature Fi....arrgh I hate you!"
But on the other hand it can give you a sense of understanding a person.
Your comment about being possessive of our Fi information was interesting. I do journal sometimes, to include inner feelings, and usually want to burn the pages forthwith because I am paranoid that someone might find and read them.
Yep, totally know that feeling, though it was more intense when I was younger, and I've opened up more as I've gotten older.