Here's why:

* Newton's Second Law: Newton's Second Law of Motion states that the acceleration of an object is directly proportional to the net force acting on it and inversely proportional to its mass. This is expressed as the equation: F = ma (Force = mass x acceleration).

* Constant Force: If the net force on an object is constant, and its mass doesn't change, the acceleration will also be constant.

Examples:

* Free Fall: An object in free fall near the Earth's surface experiences a constant acceleration due to gravity (approximately 9.8 m/s²). This is because the force of gravity on the object remains constant.

* Pulling a Cart: If you pull a cart with a constant force, it will experience constant acceleration. The force you apply is the net force on the cart (ignoring friction).

Important Note: Constant acceleration doesn't mean the object is moving at a constant speed. It means the object's velocity is changing at a constant rate.