The chemical is capsaicin, and it’s a neurotoxin. That’s why with repeated exposure you can start to gain resistance.

Birds are less sensitive to capsaicin, which made having it around seeds beneficial for plants. Birds eat the seeds and then spread them after flying somewhere else, but mammals are deterred from eating the seeds and they are poor vectors for spread because most mammals that eat large amounts of vegetable matter have molar teeth that aid in breaking down small seeds for digestion.

Until a certain mammal decided they liked the burn, and deliberately spread those plants further than birds ever did.