In Buddhism freedom from suffering is regarded as the natural state, and so is wisdom and compassion. These are not learned or acquired attributes from the outside; they are the essential nature of all beings and all conditions.
How is this known? It is known through the law of kamma, the nature of cause and effect. It is the Buddhist process of retribution.
Why do we feel unhappy? It may be because we have said or done something which has made us feel like that. We have said something to someone which we now regret. We find that we feel bad about it. We may try to justify it to ourselves, but we cannot and there is a vague sense of shame, guilt. This feeling is the result of what we have done. It is the kammic reaction. If we had not Intended any harm to another, we would not be feeling like this now. Kamma is quite simple and obvious in that sense. That is instant kamma, instant retribution. There may be delayed karma. Perhaps we did something years ago as a child, the results of which are just beginning to manifest. Perhaps we took a spider and pulled all its legs off. Perhaps we told a lie and got someone into trouble. At the time it did not feel too bad, but the mere thought of it now makes us cringe. That feeling, again, is kamma at work. The same is true in reverse - selfless actions bring good results. One may feel good about oneself, not in a narcissistic way, but there is a sense of feeling happy about what one has done. These are simple examples, but the principle can be seen running right through everything we do. Some regard it as a power which is absolutely exact and precisely equal. I steal from you; someone else steals from me. I stab someone in the back, literally or metaphorically; someone does the same to me.
All beings are the owners of their deeds, the heirs of their deeds their deeds are the womb from which they sprang, with their deeds they are bound up, their deeds are their refuge. Whatever deeds they do - good or evil - of such they will be the heirs.
Not every difficulty in life is the result of bad causes, of course. We cannot make judgements when it comes to other people. Many things appear bad in the beginning, but turn out to be beneficial in the end. We develop and mature because of them. Wealth is not always good; poverty is not always bad. Illness is not always bad. We all get ill; it is what we do with it that matters. Once we become aware of the principle, see it at work within our lives, then we realize that kamma is actively righting wrongs. These are not wrongs decreed by human beings in a court of law; these are wrongs decreed by nature. The laws of nature seem to be frighteningly just. Of course, there are many injustices in the world which are hard to explain in this way. But the law of kamma is not simplistic; it is very complex, and definitely not to be used as a judgement of others because we do not see the full picture.
We can only know kamma for ourselves; it is to be recognized as an active and powerful force within oneself, and then it can be trusted and used as a means of guidance in one's life. In the same way that kamma may take months or years to come to fruition, so too, in the eyes of many Buddhists, the results of our actions may come in future lifetimes, or we may be now experiencing the results of something we did in past lifetimes. On a very materialistic level, the tendency is to believe that generosity, for example, in this life will bring riches in the next. But the Buddha was concerned with the spiritual path, not a materialistic one, and his teaching was related to wisdom and compassion, sorrow and liberation from sorrow, not on gaining success or riches in future lives.
How people treat you is their karma; how you react is yours.
I believe in Karma. If the good is sown, the good is collected. When positive things are made, that returns well.