The problem of pain/ suffering in the world is often regarded, by modern Western people, as a decisive argument against the reality of the loving Christian God*.
...On the lines of - "a loving God would not allow such pain and suffering as we see in the world" - typically followed by an historic (or current) instance of (what is assumed to be) extreme and widspread suffering.
It is worth noting that this was not seen as a problem for the Christians (including converts) of the past - who had a great deal more of pain and suffering to contend with.
Nonetheless, the important thing is to answer this for modern Western people.
(The problem of pain is, in fact, properly, a sub-division of the problem of the existence of evil in a world ruled by a loving God; but modern secular people do not have any understanding of evil - and in practice tend to reduce evil to 'whatever causes suffering'.)
My own perspective on the problem of pain begins with my baseline metaphysical assumptions concerning the nature of reality. These are not shared by most Christians; but they may be of help to those who are not convinced by more mainstream orthodox Christian explanations - such as CS Lewis's book The Problem of Pain.
To begin with I regard God to be my loving Father, and the creator of this reality; but I do not regard him as omnipotent. Indeed, I think that it is the habit of thinking in abstractions such as 'omnipotence' as a root error that leads to fallacies such as 'the problem of pain'.
I regard my life, my continuing life, as being for the purpose of spiritual development (theosis): that is my life is about having experiences from which I am intended to learn. And pain is one of the experiences of my life.
I think it is absolutely vital to regard this matter from an individual and specific point of view.
If we instead asked what is the reason for pain and suffering in an abstract manner - which includes many possible kinds of pain, and includes all people (past, present and future) who are asserted to have suffered pain (either or both extremely or in large numbers) - then we have simply created an unanswerable question.
The question is so badly formed as to have - in principle - no possible answer.
When we do not properly know what we are asking, we would not know whether or not we had answered it.
We would not even know whether - or to what extent - the abstract persons actually did suffer pain - and we have no solid way of finding-out. Nor is there any way of measuring or adding-up pain.
Another of my assumptions is that God is my loving parent, and as such I am one of a world of his children; each of whom is loved as an individual, and each of whom will have different experiences and things they need to learn from living.
I cannot possibly know why 'other people' - whether as individuals or as categories - experienced what seems to be pain or suffering.
Because the truth of the matter is that there are going to be literally billions of true-explanations for why pain and suffering happened on (?trillions) of occasions.
(Yet people who ask such questions seem to expect a snappy answer - in a few sentences!)
The proper question is: What about my own pain? And perhaps also the pain of that handful of people who I love and about whom I have great knowledge?
Can I know why I suffer, and why those I love suffer?
Yes, I believe this can be known. It can be discovered in the ways that individual Christians may learn about God and his hopes and plans; by means of reflection and prayer.
This may not be easy - since typically we are confused, distracted, and our motivations are often muddled or base. But insofar as we are able to clarify and purify our seeking of an answer - we can receive one.
This answer will be a direct-communication from God understood by a single act of comprehension; which means that it will not depend on language - which means that it will not be fully or un-distortedly state-able in words.
Even less will that wordless understanding be communicable to other people.
None of which matters.
What we need is a personal understanding of specific instances of pain and suffering in ourselves and other specific beloved persons. If we receive such knowledge directly, and understanding it as such - our question has been answered.
And that is the answer to 'the problem of pain' - when the question has been properly understood and conceptualized.
*I have frequent occasion to reflect on this matter since I suffer from frequent and prolonged episodes of that purest of severe pain disorders: migraine. I call migraine 'pure' pain because the disease is (pretty much) 'just' pain, and can be extremely intense and last up to a few days; and the conditions may persist for many decades. The pain in migraine is not perceptibly 'functional' unlike most causes of pain. It doesn't seem to signify anything other than itself. Migraine therefore seems singularly 'pointless', almost like torture for its own sake - or the sake of being cruel. Hence migraine might (as much as almost anything) be expected to lead to inferences of the malevolence of some supernatural being.