There are different types of peppers. Some are sweet and some will set your mouth on fire! Spicy peppers contain something called capsaicin, a compound which has a strong burning taste.
We all have a different number of taste buds, so people react differently to hot food. On average, the human tongue has 2,000 to 8,000 taste buds. Each taste bud has between 50 and 150 taste receptor cells! When you bite into a pepper, capsaicin sets off pain receptors in your mouth, telling your brain you’ve eaten something hot. A scientist named Wilbur Scoville figured out a way to measure this heat, and the spiciness of peppers can be tested on the Scoville scale.
But taste sensations vary, within a single taste bud and on the whole surface of the tongue. So that's why you might be able to chew a chili without breathing fire, but your friend hates peppers on their pizza!