i think it's really interesting, the idea of depression perhaps sometimes being related to the weaker side of an internal loop giving out. for an ENFP the introverted loop is Fi-Si, as fidelia explained well, and for us the weak partner is Si. i would tend to agree that when i get wrapped up in how-things-were-and-how-great-it-was (Fi-Si looping), it leads to thoughts and behavior that could easily correlate with or turn into depression, especially when the emphasis is the factors in the past that made it great (Si) and not the feelings (Fi). it can lead to despair that things are that way no longer and questioning how they could ever be again. the key to breaking out of it, of course, is getting Ne and Te's butts back in gear (seeking out hugs from someone with Fe is nice too
) - and Fi-Si looping can be very useful in reestablishing things that you enjoyed and appropriately integrating traditional elements and meaning into new things. it is simply, like any other loop, dangerous when it gets out of balance and takes over.
i am curious to hear more about Ni-Fi looping. i imagine it's taken place in my mind at some point, but i wonder when. perhaps in seeing the self in new ways?