Here's why:

* Acceleration is the rate of change of velocity.

* Velocity is a vector quantity, meaning it has both magnitude (speed) and direction.

* When a body comes to rest, its final velocity is zero.

* Since the initial velocity was non-zero, and the final velocity is zero, there is a change in velocity.

* This change in velocity, happening over a period of time, means there's acceleration.

* The acceleration is negative because it acts in the opposite direction to the initial velocity, causing the body to slow down and eventually stop.

Example: Imagine a car moving forward (positive velocity). When the driver applies the brakes, the car slows down and eventually comes to a stop. The brakes cause a negative acceleration (deceleration) that acts against the car's forward motion.