Brains organize things into familiar patterns, which are different for different people. This can make communication tricky, so it's useful to conceptualize these patterns and use them to help translation efforts.
Crystals are nifty things! The same sort of crystal will reliably organize in the same pattern, and always break the same way under stress.
Brains are also nifty things! The same person's brain will typically view everything through a favorite lens (or two), and will need to work hard to translate input that comes in through another channel or in different terms. When a brain acquires new concepts - even really vital ones - the new idea will result in recognizeably-shaped brain-bits. Different brains, therefore, handle concepts differently, and this can make it hard for us to talk to each other.
This works on a number of levels, although perhaps the most obvious is the divide between styles of thought on the order of "visual thinker", "verbal thinker", etc. People who differ here have to constantly reinterpret everything they say to one another, moving from non-native mode to native mode and back with every bit of data exchanged. People also store and retrieve memories differently, form first-approximation hypotheses and models differently, prioritize sensory input differently, have different levels of introspective luminosity1, and experience different affect around concepts and propositions. Over time, we accumulate different skills, knowledge, cognitive habits, shortcuts, and mental filing debris. Intuitions differ - appeals to intuition will only convert people who share the premises natively. We have lots in common, but high enough variance that it's impressive how much we do manage to communicate over not only inferential distances, but also fundamentally diverse brain plans. Basically, you can hit two crystals the same way with the same hammer, but they can still break along different cleavage planes.
This phenomenon is a little like man-with-a-hammer syndrome, which is why I chose that extension of my crystal metaphor. But a person's dependence on their mental crystallography, unlike their wanton use of their hammer, rarely seems to diminish with time. (In fact, youth probably confers some increased flexibility - it seems that you can probably train children to have different crystalline structures to some degree, but much less so with adults). MWaH is actually partially explained by the brain's crystallographic regularities. A hammer-idea will only be compelling to you if it aligns with the crystals in your head.
Having "useful" mental crystallography - which lets you comprehend, synthesize, and apply ideas in their most accurate, valuable form - is a type of epistemic luck about the things you can best understand. If you're intrinsically oriented towards mathematical explanations, for instance, and this lets you promptly apprehend the truth and falsity of strings of numbers that would leave my head swimming, you're epistemically lucky about math (while I'm rather likely to be led astray if someone takes the time to put together a plausible verbal explanation that may not match up to the numbers). Some brain structures can use more notions than others, although I'm skeptical that any human has a pure generalist crystal pattern that can make great use of every sort of concept interchangeably without some native mode to touch base with regularly.
When you're trying to communicate facts, opinions, and concepts - most especially concepts - it is a useful investment of effort to try to categorize both your audience's crystallography and your own. With only one of these pieces of information, you can't optimize your message for its recipient, because you need to know what you're translating from, not just have a bead on what you are translating to. (If you want to translate the word "blesser" into, say, Tagalog, it might be useful to know if "blesser" is English or French.) And even with fairly good information on both origin and destination, you can wind up with a frustrating disconnect; but given that head start on bridging the gap, you can find wherever the two crystals are most likely to touch with less trial and error.
1Introspective luminosity (or just "luminosity") is the subject of a sequence I have planned - this is a preparatory post of sorts. In a nutshell, I use it to mean the discernibility of mental states to their haver - if you're luminously happy, clap your hands.
I'm reminded of a set of papers (here's one of them) I've encountered, which treat human language in an rather interesting fashion. In their model, different individuals have different conceptual spaces, representations of all the concepts and information that the individuals know about. Language maps areas of this conceptual space to individual words and sentences. Communication fails when the words you use map to different areas in your and the other person's conceptual space.
As a way of illustration, see this image. The two images are the conceptual spaces of two people, with the lines marking the areas that different words map to for the two. As they put it in the paper:
Personally, I've noticed that there's a great many deal of disagreements that vanish completely after I and the other person finally get our mappings in synch and realize that we agreed all along. After running into this problem several times with some close friends, I've come to the conclusion that I need to apply the heuristic "if somebody seems to be saying something that just doesn't make sense, find out if he's really saying what you think he's saying" more often.