Why? Because adding any number and its opposite is always going to be zero.
Consider adding the number 5 to its opposite -5
5 + (-5) = 5-5 = 0
The same applies to fractions as well. All we do is change the sign from negative to positive, or vice versa, to get the opposite number. The opposites cancel out more or less.
You can think of it as going up 5 floors (positive 5), then going back down 5 floors (negative 5). So overall you didn't change floors at all (represented by 0)
