Why do you think Jesus sweat blood while praying?

Jesus sweat blood while praying because of the anguish he felt over this impending death. When someone sweats blood, it's called hematidrosis. Extreme stress can cause tiny capillaries in the sweat glands to rupture and this causes sweat and blood to mix. This makes the skin extremely sensitive and so Jesus would have felt even more pain than normal when he was beaten before his crucifixion. See http://www.apologeticspress.org/articles/2223 to read more. Hope this helps. Blessings... (Also, the book The Case for Christ does an Excellent job at explaining what the crucifixion would have been like for Jesus pp.258-267.)