Why is it impossible to say that there are exclusively positive or purely negative environmental factors for the organism?

Each factor has its own range of values. Each organism has its own optimum, these are the values ​​of the factor at which it is comfortable.

For example, temperature is one of the abiotic factors that affects absolutely all organisms. For some species, high and low values ​​of this factor are destructive, for others, on the contrary, it is more comfortable when the temperature is low or vice versa.

Another example is predation (biotic factor). If there are too many wolves in the forest, then there will be fewer hares. If there are few wolves, there will be a lot of hares, because of the great competition and lack of food, they will begin to die. With an average value of the factor, the number of hares will be maintained at a constant level.