If you are really serious in wanting to understand why a wholly-loving God permits suffering in this mortal world (and are not merely using the questions rhetorically, as an excuse for disbelieving God and thereby enabling some sin which you greatly desire to do); then the first step is obviously to recognize that God's shaping of this mortal world is aimed at the eternal benefit of those who accept the offer of Jesus Christa and choose to become resurrected into Heaven.
This world is 'for' the encouragement of Men to accept this offer of salvation into Heaven. Life is for the 'education' of men - so that we may learn what will benefit us eternally.
There is no single cause for suffering - and suffering may be wholly motivated by the spite and sadism of beings (human, demonic) who are committed to evil.
But suffering is also sometimes some kind of a 'lesson' for someone; an example of God's 'tough love' in trying to teach those who have refused to learn from more gentle lessons.
But this cannot be known by trying to discover The Cause Of Suffering in some large (often abstract) group or classification of people (such as are beloved by the Left).
God loves us as individual and unique persons - therefore suffering that derives from God's tough love (when it happens) will be directed at the specific circumstances of a specific person.
To infer this cause of suffering requires specific knowledge of that person, and it probably requires that we have a genuine personal care for him; because only when we want the best for him is it likely that we can understand the motivations of a God who also wants the best for him - but to an even higher degree and across a longer timescale.
Most of the people in this world do not seem to want eternal life in Heaven - even if they believed Heaven to be true, they would not want it. And any person is able to make that decision to reject God's will. But God desires that such rejection be a conscious and informed choice.
A specific person may need to suffer the consequences of his choice to sustain the side against God - i.e. the side of evil; because all choices have consequences - and he may learn from these consequences.
I seem to see a great deal of suffering nowadays which comes from the choice Not to recognize evil; the choice to go-along-with - and support - the agenda of evil.
I seem to see many individual people who have made these choices, and who have personally suffered great misery, physical suffering, even death as a consequence...
And yet I see that many of these people continue Not to recognize the evil motivations that have led to their suffering.
This shows how difficult it is to teach some (most?) people; and how resistant people can be to learning the lessons which life provides.
I seem to see God providing many people with life lessons - some sweet and delightful, others tough and harsh... and yet people simply Will Not Learn from them; but will explain-away the obvious lessons by more and more convoluted and implausible scenarios that serve to sustain their earlier evil choices.
For example - someone refuses to recognize the obvious (i.e. incoherent, changing) lies of an obvious liar (incoherent explanations, rapidly changing explanations)... That person chooses to believe the obvious lies of an obvious liar.
That person then suffers very personally from his choice of evil - the lies lead to harming people he loves, the consequences harm himself.
Obedience to evil always has outcomes - and a refusal to learn from these outcomes, a refusal to recognize evil, leads to more evil in the 'cover-up' and opens the possibility of greater evils to come (and the failure to recognize and learn from them).
But there are those who do wake-up to evil when the consequences become so severe or numerous as to be undeniable.
It can't be predicted exactly what will work for an individual - it may seem trivial, or it may seem like another consequence in a sequence; yet it is the straw that breaks the camel's back.
So, as the consequences of evil multiply - more people do awaken and recognize the evil. ...Not so may as I would hope, and some of those I most hope-for are among the unawakened.
And some people are astonishingly resistant to learning (some, apparently, resistant unto death).
Yet I think I can see what God is doing in some cases. I can sometimes understand - very specifically - why God allows these kinds of suffering.