The Lack of Sufficient Understanding.

There are over six billion people in the world. People get born and die every day. People get diseases, people get married, people get jobs, people suffer and feel bad.

But no matter how hard you think about it, you'll never know how they feel.

Think about your own life. All the complicated things you do that you take for granted, every tiny little detail. Now think that even if you tell someone, they won't comprehend it the same way that it happens, or won't figure out the level of detail behind it.

Think about your friends. Think about how you see them, and how they present themselves. Now think about if they told you something had happened to them. Try and put yourself in their shoes and see how it feels. You can't do it. Know why? Because even if it happened to you, it didn't happen the same way as it did to them, and you won't understand it the same way.
You only know as much about someone as they let you know.
They may present themselves as a strong person. You'll never consider the possibility that they might still cry themselves to sleep for some unknown reason.
Think about the people you see walking around, bad make-up smeared all over their face. Maybe you call them "posers goths" or something. They're not "posing" for you or anyone else. The reason they do that is because maybe they're into that "scene" and just not good with make-up. Maybe they copy someone elses "style" just because they like it, and want to have it too.
Think about all the emo kids that write shitty songs about getting dumped by their girlfriends. You'll listen to it and just think it's some shit song by a band you hate, whereas the guy that wrote it took something that hurt him and used it to create something to help deal with it.
Think about actors and actresses. You see them on screen for a few hours and then suck in everything else you hear about them. I don't go in for all that gossip stuff, but Brad Pitt broke up with that chick from "Friends" a while back, and people blamed Angelina Jolie I think. Well it's not as simple as that. If someone breaks up with someone it's because they're not happy in the relationship (again, think about every little detail that goes behind every action you have and ever will make)

You're reading this and thinking it just sprung out of nowhere suddenly. You don't consider that it's 07:08am and I haven't slept. Or that I was lying in bed since 06:00ish thinking how to word every sentance of this post, and how I then got up, got dressed, turned on and waited for the computer to load, sat down, and began to compile this post, or that I don't care what'll happen in the aftermath of it.
If I said I can't get a job, someone would no doubt say "try applying for places." They don't get that I've tried and been rejected over and over and over, and I get tired of it, and that I have a deep fear of rejection.

I've gone around for a long time saying "Oh, people suck, " or "Oh, people are dicks." Which in some cases I still believe. But in most cases people are dicks because of something (no, that doesn't mean you, or I, have to start liking them). Something makes you the way you are, and you're never comprehend that in those six billion+ people that there are more details in their life that you, and I, couldn't imagine.
It's like animals. I have a cat. If it miows, it means it's hungry or wants to go out. If it purrs, it means it's happy. You never consider that cats might think a hell of a lot of interesting stuff too (granted that's because it's not a human, and I'm not about to get philosophical about the thoughts and feelings of animals and trees, etc, it's just an example).

This sort of lack of knowledge for others feelings and thoughts can make or break a relationship in some cases. Someone can express their feelings for you and you may not entirely believe them, due to paranoia or whatever reason you have. You may not talk often enough so neither of you know truly how the other feels. This lack of trust and knowledge can start to dissolve a relationship, and eventually destroy it, if the fundamentals behind the relationship aren't strong enough to hold it together.
This isn't the case for all relationships, of course, but you never know, you may experience it sometime. Maybe you have already.

So in conclusion; You'll never be able to understand the complexities behind every human being. Even the close ones.