Encouraging Missionaries

One of my "pastimes" is writing to missionaries -- which usually includes words of encouragement. It is well known that support from afar is important to missionaries. However, I find the apparent disregard of many missionaries for such support breathtaking. OBSERVATION: So what is it? Perhaps they are too busy to give courteous responses -- if any responses at all. Perhaps they are too tired. Perhaps they have all the support that they need thank you. Perhaps they are self-sufficient. Perhaps my encouragements discombobulate them. Or perhaps today's missionaries are just plain thoughtless. It is a mystery. (There are happy exceptions).