Because of the presence of sin in the world, there is suffering. Some suffering comes from one's own sin. Prison, addiction, and disease are some results of a person's sin. There is suffering that comes from these results.
But suffering can come from other people. There was a lot of talk about bullies recently. Some children were being picked on by bullies at school. One set of children were suffering, because of the attitudes and actions of other children.
Suffering could be a result of being different. At one point in this country, one type of person enslaved another type of person, because of differences, and economic gain.
Generally, when people suffer, it is something we should be sad about. But Peter points out one exception. If a person suffers, is bullied, because of being a follower of Jesus, it is something to be glad about.
Peter makes an assumption. The person, who is a follower of Jesus, who is being bullied, or persecuted, is living in such a way that demonstrates Jesus' character. If people can see Jesus in your life, and Jesus is being broadcast to the world, and you suffer as a result, rejoice that God is working in you, and through you, and people are against you, because you are more loving, more generous, more peaceful, and joyful. God's work in your life is a good thing.
If we respond in anger, cursing, and other fleshly actions, God's character is not moving in us. Not something to rejoice in.