Christians need to spread the gospel, because God told us to. But we should not do this with a grumpy heart, as if out of compulsion, but ratherjoyfully. We should be glad to share the good news with others that someone else once shared with us first. God will reward us if we serve us willingly.
For if I do this voluntarily, I have a reward; but if against my will, I have a stewardship entrusted to me. (I Corinthians 9:17 NIV)