Asking why you should share your faith is like asking why you should breathe.
When you stop breathing, your brain is deprived of oxygen flowing through it, and it stops functioning. When you stop sharing your faith, you are deprived of God’s love flowing through you, and the Joy of the Lord stops functioning. For when you bear fruit … God Says My Joy will remain with you and your joy will be full. (John 15:8-11)
It’s been said that God’s Love is the only gift you can’t keep unless you give it away. God mandates that you make the sharing of your faith your highest priority. In fact, He says it’s your “First Work” and calls you to Repentance when you don’t do it. (Rev 2:5)