Question

why do we eat more and breathe faster after an exercise

Answer 1

because there is a deficiency of oxygen

Answer 2

We breathe faster when we exercise to increase the amount of oxygen available to our bodies.
Every cell in your body needs a special gas called oxygen, that allows the cell to get energy from the food you eat.
Oxygen in the air is brought into your lungs. Thats when your blood picks it up and brings it to your cells.