Why does suffering exist?
There are numerous reasons why suffering happens to people:
Sufferings that come to us from God
Some sufferings actually comes to us from God! These sufferings are always for our perfection and training. The more often you put gold into the fire, the purer and more perfect it becomes! If you consider history you will find that the greatest people have always suffered the most, and this suffering is not a punishment, it’s a gift. For example your father might punish you to stop a bad behaviour from developing, or he might put you through hardship so that you are trained and properly develop and reach your potential. These sufferings are because he loves you and he wants you to grow correctly, not because he is unkind or hates you!
Sufferings that are caused by ourselves
There are also other sufferings which are caused by our own bad choices, for example if a person eats too much they will ruin their digestion and become ill! Or if a person ignores the advice of a doctor he will get sick, or if a person ignores the advices and commandments of God he will suffer in his life as a result. These are unnecessary sufferings which are our own fault. This suffering however causes us to reconsider our decisions for next time and hopefully not repeat the bad choice again!
Sufferings that are caused by others
There are also sufferings which are caused by the bad choices of other people. When these kinds of sufferings happen to us it’s an unfair injustice to us, and God has promised that he will never overlook an injustice, and will right the wrongs, and in either this world or the life to come He will compensate the poor victim of the suffering and punish the person who caused the suffering. God promises this:
O OPPRESSORS ON EARTH! Withdraw your hands from tyranny, for I have pledged Myself not to forgive any man’s injustice. This is My covenant which I have irrevocably decreed in the preserved tablet and sealed with My seal of glory. - Baha'u'llah
Do you know why these injustices and sufferings caused by people’s bad actions exist? Do you know why it’s even possible? It’s because God has gifted human beings with free will, so that we could enjoy life as humans with choices (otherwise if God didn’t give us free-will then we wouldn’t be humans! we would instead be like stones, or plants that have no free will and can’t make any good or bad choices!).
Because we have free will, it means we can choose to do the right thing, or choose to do the wrong thing and cause suffering to ourselves and others. If we use our free-will to do bad things this is not God’s fault, it is the person’s fault who chooses to do the wrong thing. If someone does do the wrong thing with their free will, God is aware of this, and will compensate the victim, and punish the perpetrator, either in this world, or the next. This is His promise to us.