Question

How is the excessive use of fertilizers and pesticides harmful to us?

Answer 1

Because many pesticides can be harmful, not only to the target pest but also to humans, pets and non-target organisms -- including beneficial ones -- is something to be avoided. Most pesticides are non-selective, meaning that, not only is the target pest being affected, but that other organisms -- including many that you might not intend to harm -- suffer as well.

Answer 2

they can harmful to the crops.
they can destroy homes of bacterias and other animals living in the soil. this can lead to soil erosion.