Because it is usually something you have no interest in doing...
When God was dealing with me to preach, to say the very least, I was hesitant. the Lord "spoke" to me and it was as clear as an audible voice although it wasn't. I asked God to prove it (fleece theology, don't recommend it). In the next week three almost complete strangers walked up to me and said the EXACT words at three different time and places that knew NOTHING about what had happened. One of the people I actually named in my challenge to God. After the third time, I have never doubted my calling again.
For what mortal has ever heard the voice of the living God speaking out of fire, as we have, and survived? ~ Deuteronomy 5:26
If you're not prepared to risk your very life for your "enemy" you have no right to speak to him of love. ~ Daughter
Many say they are called... but I am pretty convinced that with many of them it was the wrong number. ~ Project Peter