Karma.

@ CallusedSilk
Like I said before, karma's not supposed to be good by default. Karma's what you make it. You said "someone to learn their lesson from how they treated me in this lifetime" which implied you want them to learn their lesson for any bad things they've done to you. So how exactly would they "learn their lesson" without suffering on some level? That phrase also has a pretty negative connotation to it. Yes, good people are punished because they weren't always good people. Not everyone is brutalized though. Again, that's coming from a select view point. That's how some people on this world think. Buddhism is about compassion, we're not going to point our finger at you and think you were a totally awful person in another form because that's hypocrisy and defeats the purpose of being Buddhist. No one will know why something happened to you unless you remember your past lives but again, totally different topic. The whole point in karma being based upon your past actions to eventually "learn your lesson", you make yourself learn it though, not other people spending their life hoping you do. No one can affect other's karma like that.

@ Xiaholic
Buddhism is definitely a fascinating religion, and one you could spend a life-time studying.
I'm actually fairly ignorant on a lot of it, despite my research. The only religion I was exposed to growing up was (and still is) Catholicism and Christianity in general.
I first became exposed to it when I started looking for a religious identity. I was bouncing around between Secular Humanist and agnosticism when I came upon Buddhism, and I was shocked at how much I connected with the religion. I was under the impression that Buddhists believed in souls, reincarnation and all that as the Eastern religions are grossly misunderstood in Western society.
The suffering teachings definitely boggled my mind as well. It was basic common sense, but you could spend a life-time trying to come to the same conclusion.

After doing more research on karma since by original post, I have found that Karma is in fact cause and effect, at least in a Buddhist context. Buddha taught that we are all connected in a stream of consciousness and that every action (the cause) can have far-reaching consequences (the effect). The Big Bang was the First Cause and the growth of the universe was the effect it had on the world. (Well, as for the Big Bang, Buddha taught about a "First Cause" similar to the Big Bang, but also taught that such questions considering the origin of the universe, existence of god(s) etc. are unanswerable and will only cause suffering as we get angry when we can't answer these questions. Men search desperately for the truth. Buddha taught that religion stems from man trying to answer life's big questions)
Our personal karma is the kind of "energy" we send out. As in, if you do bad things, think negative thoughts and have more negative emotions than positive ones, you will have negative karma. However, if we do good things, think positively and have more postive emotions than negative ones we will have good karma.
Some people do believe in the form of karma in the context of your friend, but I personally think that that is incredibly unfair. Should someone be punished for something they don't even remember doing?
Buddha taught the doctrine of rebirth (which many Buddhists are agnostic on, myself included) that our karma, such as our actions, thoughts and feelings carry over from one life to the next, when our karma is reborn in another body instead of reincarnation, where the soul is reborn. Our karma being reborn means our tendencies are reborn. If we have good karma, we will be reborn on one of the nine planes of existence.
The Nine Planes of Existence basically go like this; if you long for love, you are a Longful Ghost (a plane of existence) and a longful person condemned to suffer. There are more, but I haven't researched them too much. I believe in it in the sense that we are never the same person from one moments to the next, another Buddhist doctrine. One minute we are longful, next we are violent and destructive (I believe being a destructive person means you live in the "Demon" plane of existence).
It's definitely a lot to take in! ;D

@ Angels and Roses
Wow, it gets even more interesting and deeper than I thought! Thank you so much for sharing that with me I agree about Western society being quite oblivious towards Eastern religion, they hardly taught a lot about other religions at school - they'd go over some briefly, but the main one was Christianity. One thing which upset me about school was that we were hardly taught anything about Asia, in honesty.

Being quite a pessimist myself, I just don't think I have it in me to ever become a devoted Buddhist; I find positivity easily tiring, but I'm really ashamed of that because I used to be told I was the one who "always smiled", "made others feel better" etc. But then something changed as I grew up and left school.

"Buddha taught that religion stems from man trying to answer life's big questions."
I love that part! Sounds so true and in a way quite inspiring. I also love that Buddha teaches not to stress oneself with finding unanswerable truths, that's really cool.

I hate to be even more of a nuisance, and please don't feel obliged to dig into it if you don't know already, but I just wondered if you knew what happens or is taught to Buddhists who might have lost that positivity? In what ways are they encouraged to find that positivity once more?

@ Xiaholic
http://www.buddhanet.net/tib_heal.htm
This should help somewhat! ;)

Meditation and reflection is a big thing. Buddhism teaches that our mind can be very harmful to our well-being, and that we can punish ourselves for completely human things that are in our nature. They teach meditation, which is extremely calming, can send our mind into a state of happiness and clarity (I remember I had a very clear moment once, and the feeling is indescribable. It's very beautiful, though, like you're complete) :)
Reflection is also very important. Buddhists teach that we should think of the now, as to avoid guilt over past actions and worry over future ones, but also that your life can flash by if you dwell too much on the future and past. The past can't be re-written, and the future is unknowable.
However, Buddhists are encouraged to reflect on ourselves. An example would be you reflecting on your lack of positivity. Where does it come from? How has it effected my life? Do I want to change? Buddhists teach about the human spirit. Humans are beautiful creatures, and even though it mightn't seem so, can accomplish great things. The first step is defeating our egos.

Also, if you want to talk more, definitely PM me! ;) Sorry for not getting back to you sooner. I have a tonne of exams around the corner and have been studying like crazy :(
Also, the link I posted is written by a Tibetan Buddhist. I myself am a Zen Buddhist, the most liberal and relaxed form, so I wouldn't know too much about their school. Most of I what I know stems from Zen teaching :D