Why did Jesus sweat blood?

Jesus was in such agony in the Garden of Gethsemene that his sweat became like great drops of blood. What was causing him such anguish? Was it just the thoughts of crucifixion? Others had faced crucifixion with less angst and Jesus had told people not to be frightened of those who can only kill the body (Luke 12:4), there was something else, find out what here: 

You can read Luke 22:39-36 here.