why do people chill when he feel cold?

Answer 1

It is the body's natural instinct to try to make more heat. It is an uncontrollable instinct just like blinking or our eyes closing when we sneeze.

Answer 2

The two reactions that come to my mind are shivering and goosebumps.

Shivering contracts muscles in opposition to each other. This uses up energy and produces heat without much movement.

Goosebumps are a remnant from when our evolutionary ancestors had fur. Each hair follicle on your body has a tiny muscle attached. When an animal gets cold, their muscles contract and raise their fur on end. This traps air underneath and provides an extra layer of insulation. Humans no longer have enough hair for this to be useful, but they still possess the tiny muscles involved. When they get cold, their hairs stand on end and pull the skin slightly as well, causing goosebumps.

Another part of the chills is contraction of the skin. When a person gets cold, the capillaries close to the skin constrict, reducing blood flow and forcing warm blood into the core of the body (since it is more important to maintain warmth inside the chest than it is, say, in the arm or close to the skin). The decrease in blood flow causes the skin to contract slightly and also produces a cold sensation in the extremities.