Here's why:

* Gravity: The primary force acting on an object near the Earth's surface is gravity. Gravity always pulls objects downwards towards the center of the Earth.

* Acceleration due to gravity: This downward pull of gravity causes an acceleration of approximately 9.8 m/s² (meters per second squared).

* Upward motion: Even though the object is moving upwards, gravity is still acting on it, causing it to slow down. This slowing down is also considered acceleration, but in the opposite direction of the object's motion.

* Negative sign: We use a negative sign to indicate that the acceleration is in the opposite direction of the object's upward motion.

Important Note: This assumes no other forces are acting on the object, such as air resistance or thrust from a rocket.