Sin have taken over the world and is a major reason people suffer tribulation. In the book of Genesis when man sin against God, pain, sorrow and suffering entered the world, since then suffering have been the order of the day. Sometimes pain and suffering comes because of the decision we took and sometimes it is due to our fellow humans ruling us in this world. Sometimes God allow us to suffer and pass through hard times just to rest our faith and also draw us closer to him. During the time of tribulation, it doesn't mean God have abandon us, but he wants us to come close to him.
“In the world ye shall have tribulation: but be of good cheer; I have overcome the world.” (John 16:33)
Sometimes we suffer tribulation because we embrace truth instead of lies. The bible said that those who follow the commandments of God will likely face rejection and torture from our fellow humans. God allows us to face tribulation because it strengthens our faith and hope in Him. When we face tribulation that's the time we will know our faith in God. Those who don't trust in God may likely fail the test at that time.