I think there is also something in
-leading (in ILIs, not so much in IEIs) that lends itself to pessimism. You have so much "perspective" that it's easy to start seeing everything as pointless.
As Dynamic types (a Reinin dichotomy I think is actually worthwhile, fwiw), ILIs tend to see the world around them as constantly changing, and as
types their awareness of change is of a comparatively long timeline. Everything is seen as transient (I forget who wrote it but one of the ways in which the ILI is supposed to reassure the SEE is in reminding them that "this, too, will pass"), which is both a blessing and a curse, a source of of both reassurance and despair.
An ILI easily gets the idea that because nothing stands the test of time, everything is pointless anyway. Why even act?
I think I'm actually a fairly optimistic person, but even my optimism is based on this view of the world which I can only see as pessimistic.