ייִדישער מחשבֿה אויף דברים 3:25

The "affliction" of the Egyptians G-d refers to, is their very "hard heartedness." Allegorically speaking, the lesson of "He taught him about a tree," i.e. the piece of wood that Moses threw into the water, was the same lesson as that of the tree of life in gan eden. When one is aware of one's ultimate goal, eternal life, one can swallow some bitter pills in this world, serene in the knowledge that they pave the way to ultimate eternal life and sweetness. Just as a bitter medicine often restores a body to health, so bitter pills can pave the way to spiritual health. Just as one has to submit to the doctor's instructions, since he is the expert, so one has to submit to the Creator, who is the ultimate life giver. My medicine, says G-d, is guaranteed to help, whereas human physicians cannot even guarantee that the bitter pills they prescribe will produce the hoped for result.