R. Kent Hughes:
No one has ever known the sorrow our Lord experienced [in Gethsemane]. Luke the physician says, "And being in anguish, he prayed more earnestly, and his sweat was like drops of blood falling to the ground" (Luke 22:44).
But what was the cause of Christ's bloody sweat?
It was not the pain that caused the horror. It was not the shame. It was not the imminent desertion of his disciples. It was the fact that he was going to pay the penalty for our sins! The understanding of what the sacrifice meant, which only omniscience could bring, caused our Lord to break out in a bloody sweat (emphasis mine). It was the crushing realization of the horror that crushed him. Christ's resolve to endure the agony, even at such a great price, demonstrates his lordship and divinity.
(Taken from Nancy Guthrie's book, Jesus, Keep Me Near the Cross, pg. 32-33)