Q: You either have some of something, or you have none of something. There's no such thing as a "negative amount" of something in real life. I can have no (0) hats, or I can have 1 hat, 2 hats, 3 hats, etc. But I can't have a negative amount of hats.
A: sure, but you can have a change in the amount of hats, and this number can be positive or negative. if you have 2 hats but you used to have 5, you can say you have negative 3 hats compared to what you had before. you'll also need the negativity concept if you have debt. for example, if you have -3 hats, this means you owe someone some hats, so even if you aquire some hats later you'll have to immediately give them away before you get to keep that 4th one, bringing you to 1