"LaVerna" wrote apparently:
Why do you have to put a negative value for amount of payment and present
value?

Well, you don't. But you do need to use __opposite_signs__ for the
directions of cash flows. You can choose to use positive numbers for
"inflows" and negative numbers for "outflows", or vice versa. Which
one you choose depends on your point of view -- and which way you find
more convenient at the time.

As to the reason why: the answer lies in the algebra of the math
formulas.