Why did robert e lee chose to fight for the confederates instead of for lincoln?

This i do not know sadly but i did some research for you and google says and i quote

"When Virginia declared its secession from the Union in April 1861, Lee chose to follow his home state, despite his desire for the country to remain intact and an offer of a senior Union command. During the first year of the Civil War, Lee served as a senior military adviser to Confederate President Jefferson Davis".

answer; i believe the correct answer is (the willamete valley);