Making Sense of Suffering, adapted from the speeches of Rabbi Yitzchak Kirzner, who, in turn, seems to have adapted the large majority of his work from the Ramchal in Derech Hashem (beginning here, mostly), discusses several reasons why one may suffer. It's important to note that these reasons aren't all mutually exclusive; a person may suffer for multiple reasons on this list simultaneously. However, the first item on the list is nearly always applicable.
- Spiritual "vomit": The same way that when one ingests a poison, he throws up in order to purge it from his system, suffering is intended as a way to purge the spiritual poison of sin from one's body.
- Gilgulim: Sometimes, particularly regarding children (i.e. under 20) who suffer, suffering is brought not because of what they have done in this lifetime, but what has happened to them in previous lifetimes.
- Alarm clock: The same way that some people are in such a deep sleep that they require a very rude awakening to wake up, suffering may be disproportionate to the sin in order to wake the person up and inspire them to repent.
- Character building: Suffering sometimes helps a person develop character traits he didn't know he had. This is what Chazal refer to as "suffering of love." Often this is specifically to imbue one with the middos of Emunah and Bitachon, of trusting that Hashem knows what's best for a person, even if we do not understand it ourselves.
- Showing who's in control: If a person is so arrogant as to believe he has 100% total control over his life, Hashem may give him suffering to wake him up to the fact that Hashem is the one in total control. "All are in the hands of Heaven except for fear of Heaven." The only thing in one's power is how he acts or reacts.
- Inspiration for others: A person may suffer not to help himself, but to help others. When others see how he suffers yet maintains pure Emunah, that serves as a paradigm for others from which they can learn and which they can model.
- Sharing the burden: The truly righteous of the generation may suffer for no fault of their own, but rather to take the suffering that their generation deserves but is unable to bear. As we are all connected, the truly righteous are able to bear the burden for us, and through their suffering, we are able to be healed.
- Physical Mechanisms: Sometimes, suffering just happens because of our actions. A person eats a fatty diet over a long enough period of time and wakes up twenty years later with clogged arteries (actually, he should be thankful that he woke up). The question here shouldn't be "Why did You do this to me?" but rather "Why did You not perform a miracle to save me?" Hashem doesn't intervene in such cases unless one is exceptionally worthy. Otherwise, suffering may be due to our own stupid decisions.
- Hanhagas HaMazal: If man is given free choice, isn't it possible that the world will never reach its perfection, as man can continuously mess up? Sometimes, Hashem will subtly intervene, in what's called Hanhagas HaMazal. Sometimes, as a result of this, people suffer. Nevertheless, they will benefit as a result, whether as one of the above or purely as reward for their participation in helping the world forward. There are two types of suffering that fall under this umbrella, which I'll term "active" and "passive."
- Active Hanhagah: In this version, a person needs to be developed in order to aid the world. In order to do so, he must suffer, to, for instance, build character (or something else listed above). However, this suffering is always attributable to some sin, though completely out of proportion to it. The book gives the example of Rebbi Yehudah HaNasi, who was punished with a severe toothache for the insensitivity expressed in his shooing away a lamb being taken to the slaughter, saying, "Go; for this you were created." The suffering was meant to develop Rebbi into the leader he would become, but there still needed to be some basis for it.
- Passive Hanhagah: Rabbi Kirzner's analogy is a son who comes to his parents with a request for a motorcycle. Concerned for his physical safety, as well as the potential spiritual damage due to hanging around negative influences, the parents refuse. The son continues pleading, offering to get the safest helmet on the market, going on a hunger strike, and eventually threatening suicide if he doesn't get his bike. What should the parents do now? He's made the bike so much a part of him that he cannot live without it. So they allow him to get the bike and hope that they've provided the proper upbringing that one day he will realize his mistake and give up his bike.
So, too, Hashem will sometimes let mankind run the world, without His intervention. Man will build a world in his image, based on his greed and arrogance. Eventually, it will collapse. "Your dream will reveal itself to be a nightmare. The bigger and stronger you build that world, the more powerfully you will experience the nightmare. Only then will you utterly reject this idol, with every aspect of your being." As a result of this world, many will suffer, not due to any fault of their own, but because Hashem has removed His influence and let nature run its own course.