Some are trying to prove their faith. Others have a sincere desire to save souls from an eternity in Hell.
Have you ever been in the grocery store and saw someone about to buy a brand of something you tried last week, and it is terrible? It is a friendly effort to spare someone from a bad dinner.
Hell is for ETERNITY. We don't want others to go there.
The best thing I have in this life is my relationship with my Father in Heaven and with Jesus and the Holy Spirit.
And.... the spirit in a man compels him to do what he does. When he is filled with the Holy Spirit of God, he naturally desires to do what Jesus did- preach the gospel, perform acts of love, cast out devils, and heal the sick. It is his natural course.
As Jesus said, "If you love me, you will obey my commands."