> A downward-growing stack is more intuitive. To access items on a downward-growing stack, you use positive offsets; for instance, sp+0x10 could be the second stack-passed argument to a function. For an upward-growing stack, you have to use negative offsets.

Why would one use negative offsets with the stack pointer instead of positive offsets with the base pointer?

Using the stack pointer, I can never dynamically push something onto the stack without changing the offsets.