Why the demand curve for investment slope downward?

real interest rate is graphed on the y-axis and quantity of investment is on the x-axis. Both values increase as they go away from the origin. If real interest rate is higher, quantity of investment will be lower, creating a point on the upper left side of the graph. If real interest rate is lower, then quantity of investment will be higher, and a point will be created on the lower right side of the graph.