I can't figure out a good way to explain it, but have you been in a situation where you were surrounded by people who were hurting themselves? Loving the ways of the world and immersing themselves in sin? Perhaps they weren't even aware of the hurt, but you knew. And it hurt, because you wanted better for them.
That's what the missionaries are surrounded by all the time. Sun worshippers--staring at the sun until they go blind. Bhuddists--begging for food in their saffron robes in an attempt to give up everything. Muslims--bowing five times a day in prayer. Dalits--scraping human waste out of the sewer as they pray for something better through reincarnation.
The missionaries are people--just like you and I. Pray for God's strength, for encouragement, for His joy, and for boldness.