I think that it is learning to try to step back and really critique, internally, WHY you think or feel something. What led you to think that, or feel that? It can be diffficult to get to the root, and on the downside, it can result in some serious over-analysis, and becoming highly confused as a result, so it's definitely a balance that can be hard to maintain at times, but I think it is really learning to critically assess the WHY. Because, as fidelia indicated, there is pretty much always a reason - it just may not be readily apparent. Also, it may be, in the end, still nearly impossible to articulate, because I think that many times our WHY is a result of a million separate threads, seemingly unconnected, but when you take them all in their entirety, they all point to this single answer.
[Separate topic, but there's also the possibility too that even though all of the threads point to this single answer, on our end there's always the possibility we're missing a thread or we're connecting the wrong ones -- i.e. it's possible we could take another set of threads and come up with a different answer. Off-topic, but this is a reason I think Ni-doms can seemingly suddenly shift perspectives... it's because they 'suddenly' are taking into account a new set of threads, thus resulting in a new Result, if that makes sense]
Specifically, with a concrete example, say you get a bad vibe from someone. You just meet them, and you 'know' they're a certain type of person. Well, ultimately there's a reason you think that, if you really dig into it. For example, if you analyze, you might realize that it's their mannerisms, or it's a specific thing they said that wasn't in alignment with some other aspect of their presentation/what they're saying, or they might drive a bmw and so you 'know' they place value in status/image on some level, and when you tie that in with the way they raise their eyebrow when they say something, you know that there's a very high probability that they're also likely to be X, Y, and Z, and therefore do AA with their life.... and so on. That's the idea. There's reasons you think something, it's just that over the years it's second-nature, or all of these interconnections happen simultaneously, mostly, in your head, that you don't think to break it down (or most of the time it would take too long to do so or is pointless/counterproductive).