The way I see it, there are a few parts to your question; I have tried to address each of them:
“Why does suffering seem so random?” I am guessing that what you mean is why does suffering seem arbitrary or unjust? Or rather, IS suffering random?
The ultimate answer is No - while suffering appears random in both its extent and who suffers, God is all-powerful, good and wise (which the Bible says he is), and nothing happens which is outside his good purposes. Sin and suffering still happens but somehow it fits in with God’s good plans and purposes. God does not cause evil or sin but he does have power over it.
When suffering is discussed in the Bible, we see several things:
1) From Creation, we see that God has a plan and a purpose. God is good, and has created people to be in a relationship with him. (Genesis 1-2) God has numbered the hairs on our heads. He is in control and cares what happens to us. (Luke 12:6-7)
2) Suffering came into the world when sin came into the world. We can’t explain it - the point of sin and evil is that it is an alien intrusion into God’s good world. This does not mean that all suffering correlates to some sin, rather, that all suffering can be traced back to that first sin, from which came sin’s penalty: death.
3) The bible recognises suffering appears random. Sometimes, good people suffer. For example, Job - he was a great, godly man, but he suffered as a part of God’s plan. The apostle Paul, who wrote half of the New Testament, went through all manner of trials, including imprisonment, beatings, and shipwrecks (see 2 Corinthians 11:24). Sometimes, evil people seem to prosper - Job speaks of this in Job 21, and the psalmist Asaph writes about it in Psalm 73.
5) Suffering does have a positive side, God uses suffering to help his people to become more like Jesus, through whom we can hope in heaven. Paul, who as we saw above, faced much suffering, writes “More than that, we rejoice in our sufferings, knowing that suffering produces endurance, and endurance produces character, and character produces hope, and hope does not put us to shame, because God’s love has been poured into our hearts through the Holy Spirit who has been given to us.” (Romans 5:3-5). You can see this in the world around us as people talk about growing through hard times.
6) The most important thing we can know about suffering is that God hates suffering and death. It is not how he designed the world. So God dealt with suffering and death by sending his own son to suffer and die for us, so that those who have faith in Jesus can have eternal life - where there will be no suffering or death ever again. God did this because he loves the world. John 3:16 says “For God so loved the world that he gave his one and only son, that whoever believes in him should not perish but have everlasting life.” The Bible describes this eternal life in Revelation 21:4 “[God] will wipe away every tear from their eyes, and death shall be no more, neither shall there be mourning, nor crying, nor pain anymore, for the former things have passed away.” This is what God wants for his people.
So that is a bit of a picture about what the Bible says about suffering. It is not ‘random’, even though we may not see why we or others have particular burdens. More importantly, in suffering (as well as other aspects of life), God is working in us to bring about his plans and purposes. It may be to test us and help us grow. It may be to help those who don’t know him to turn to him. It is sometimes hard to tell - especially in the midst of suffering. But we have this promise from Rom 8, that in all things God works for teh good of those who love him. That gives us hope to hold on!
I am not sure how to answer this question. A lady from my church has written a book about her struggles with her daughter who has a debilitating disease. The book is called ‘Love Ella’ and is written by Madeleine Witham. This may give you more insight into thinking about this particular aspect of suffering. (You can get the book at www.love-ella.com)
“What causes people to sin in general?”
Another big question! I think in some ways, the answer is ‘sin’. Sin causes people to sin. We are born sinful. It is our nature to sin. Romans 3 describes that we are all sinful, and no one is really ‘good’ or ‘innocent’ in the eyes of God.
The issues of suffering and sin are resolved beautifully in what Jesus accomplishes. We are like babies born on a pirate ship. The babies are born into an environment which is horrible and murderous and vengeful - and this pirate ship will eventually be destroyed by a just judge. The children need to be rescued. But the children themselves as they grow up take their part in murdering and pillaging. They need to be forgiven for their part in murdering and pillaging. So the children are in need of both rescue and forgiveness. This is what Jesus does. He takes the punishment we deserve so we can be forgiven for our sins, and he saves us from the suffering and certain death of this world by granting us eternal life.
It is easy to question God about suffering - and it is good to ask these questions. But it is important that we know that God, in his son Jesus, has suffered more than any other person ever did. He knows what it means to suffer and he did so for our sake.
I hope this helps you. Please come back with other questions if you have them.