REBIRTH

Rebirth is the causal link from one lifetime to the next, as the most subtle level of consciousness passes from one life to the next, like a river flowing from one place to another. 
 
The Buddha experienced the reality of causality and rebirth on his night of enlightenment. He not only saw his own lives but the lives of countless others as well. He later explained that on our long journey of life, as we wander aimlessly from birth to birth, there have been more tears shed for us than there is water in the oceans.
 
You may not believe in causality and rebirth, but regardless, like gravity, they will still be there and can provide answers as to why good people may undergo endless difficulties, while others who are selfish and uncaring enjoy great wealth and power.
 
Causality and rebirth can also explain the existence of geniuses; for example, Mozart and Rembrandt, or why one child is loving and filial, while his sibling is deceitful and ungrateful. Mozart and Rembrandt may have been creative geniuses who had strong passions for their art and who found themselves again being pulled by that karma into a later lifetime where that talent resurfaced. And siblings are more a product of their own individual past karma than of their current environment.

Those who are undergoing difficulties, despite their current goodness, had created the causes for those difficulties in their past lives and, on rare occasions, earlier on in this lifetime. Now, the conditions for that person to undergo difficult consequences have matured. They are reaping what they sowed in the past.

Those who selfishly enjoy great wealth without practicing generosity in this current lifetime are coasting along on their positive karma that they had created in the past. But their current self-centered behavior will cause them to quickly run through that karma, like a person who keeps writing checks without depositing more money into the bank. At some point, because there are not additional causes that create wealth and good fortune, their current store of positive karma will be depleted, and suddenly they will begin to experience many problems.

We do not need to know what the exact circumstances are or what the precise cause and effect is. It is the proper understanding of the general concept of the law of causality that can help us to become better people, and this is what we need to focus on.
 
Upon seeing those who are experiencing difficulties, we are wrong to simply dismiss them by thinking that their current adversities are the result of their own past actions, and thus the hardships are essentially their own fault. With right understanding, we will do the opposite. We will feel compassion for them and have the wisdom to know how to better help them.
What they did in another lifetime or even earlier in this lifetime has long passed from their conscious mind, or has been forgotten, just as our former actions are likewise long forgotten by us. This should hardly be surprising considering how little we remember of what we did only yesterday! Here and now, in this lifetime, people who are undergoing great difficulties feel that they are helpless victims, pulled this way and that by circumstances they seem unable to change or control.
We cannot just say that this is their karma and use this as an excuse to do nothing. For us to blame and dismiss them is to act out of ignorance and arrogance. Instead, knowing about the existence of suffering and causality, we can choose to be nonjudgmental and use compassion as we try to understand and help them.
 
We can think about what they are undergoing and remind ourselves that if we do not wish to find ourselves in similar circumstances, we need to practice generosity, loving-kindness, and compassion.